In Java, can & be faster than &&?
In this code:
if (value >= x && value <= y) {
when value >= x and value <= y are as likely true as false with no particular pattern, would using the & operator be faster than using &&?
Specifically, I am thinking about how && lazily evaluates the right-hand-side expression (ie only if the LHS is true), which implies a conditional, whereas in Java & in this context guarantees strict evaluation of both (boolean) sub-expressions. The value result is the same either way.
But whilst a >= or <= operator will use a simple comparison instruction, the && must involve a branch, and that branch is susceptible to branch prediction failure - as per this Very Famous Question: Why is it faster to process a sorted array than an unsorted array?
So, forcing the expression to have no lazy components will surely be more deterministic and not be vulnerable to prediction failure. Right?
Notes:
obviously the answer to my question would be No if the code looked like this: if(value >= x && verySlowFunction()). I am focusing on "sufficiently simple" RHS expressions.
there's a conditional branch in there anyway (the if statement). I can't quite prove to myself that that is irrelevant, and that alternative formulations might be better examples, like boolean b = value >= x && value <= y;
this all falls into the world of horrendous micro-optimizations. Yeah, I know :-) ... interesting though?
Update Just to explain why I'm interested: I've been staring at the systems that Martin Thompson has been writing about on his Mechanical Sympathy blog, after he came and did a talk about Aeron. One of the key messages is that our hardware has all this magical stuff in it, and we software developers tragically fail to take advantage of it. Don't worry, I'm not about to go s/&&/\&/ on all my code :-) ... but there are a number of questions on this site on improving branch prediction by removing branches, and it occurred to me that the conditional boolean operators are at the core of test conditions.
Of course, @StephenC makes the fantastic point that bending your code into weird shapes can make it less easy for JITs to spot common optimizations - if not now, then in the future. And that the Very Famous Question mentioned above is special because it pushes the prediction complexity far beyond practical optimization.
I'm pretty much aware that in most (or almost all) situations, && is the clearest, simplest, fastest, best thing to do - although I'm very grateful to the people who have posted answers demonstrating this! I'm really interested to see if there are actually any cases in anyone's experience where the answer to "Can & be faster?" might be Yes...
Update 2: (Addressing advice that the question is overly broad. I don't want to make major changes to this question because it might compromise some of the answers below, which are of exceptional quality!) Perhaps an example in the wild is called for; this is from the Guava LongMath class (thanks hugely to @maaartinus for finding this):
public static boolean isPowerOfTwo(long x) {
return x > 0 & (x & (x - 1)) == 0;
}
See that first &? And if you check the link, the next method is called lessThanBranchFree(...), which hints that we are in branch-avoidance territory - and Guava is really widely used: every cycle saved causes sea-levels to drop visibly. So let's put the question this way: is this use of & (where && would be more normal) a real optimization?
& operator is a little-known peculiarity of Java in terms of an alternative to &&, and in years of Java programming, I've never chosen to use it. Perhaps I've been overly dismissive!
verySlowFunction() note); this is about branch prediction - or should I clarify it some more? Suggestions welcome.
& even if you wrote && if its heuristics think that doing so would be a win. I have no idea if Java's compiler does the same, but it is an easy optimization and it would be a bit surprising if they hadn't thought of it.
Ok, so you want to know how it behaves at the lower level... Let's have a look at the bytecode then!
EDIT : added the generated assembly code for AMD64, at the end. Have a look for some interesting notes.
EDIT 2 (re: OP's "Update 2"): added asm code for Guava's isPowerOfTwo method as well.
Java source
I wrote these two quick methods:
public boolean AndSC(int x, int value, int y) {
return value >= x && value <= y;
}
public boolean AndNonSC(int x, int value, int y) {
return value >= x & value <= y;
}
As you can see, they are exactly the same, save for the type of AND operator.
Java bytecode
And this is the generated bytecode:
public AndSC(III)Z
L0
LINENUMBER 8 L0
ILOAD 2
ILOAD 1
IF_ICMPLT L1
ILOAD 2
ILOAD 3
IF_ICMPGT L1
L2
LINENUMBER 9 L2
ICONST_1
IRETURN
L1
LINENUMBER 11 L1
FRAME SAME
ICONST_0
IRETURN
L3
LOCALVARIABLE this Ltest/lsoto/AndTest; L0 L3 0
LOCALVARIABLE x I L0 L3 1
LOCALVARIABLE value I L0 L3 2
LOCALVARIABLE y I L0 L3 3
MAXSTACK = 2
MAXLOCALS = 4
// access flags 0x1
public AndNonSC(III)Z
L0
LINENUMBER 15 L0
ILOAD 2
ILOAD 1
IF_ICMPLT L1
ICONST_1
GOTO L2
L1
FRAME SAME
ICONST_0
L2
FRAME SAME1 I
ILOAD 2
ILOAD 3
IF_ICMPGT L3
ICONST_1
GOTO L4
L3
FRAME SAME1 I
ICONST_0
L4
FRAME FULL [test/lsoto/AndTest I I I] [I I]
IAND
IFEQ L5
L6
LINENUMBER 16 L6
ICONST_1
IRETURN
L5
LINENUMBER 18 L5
FRAME SAME
ICONST_0
IRETURN
L7
LOCALVARIABLE this Ltest/lsoto/AndTest; L0 L7 0
LOCALVARIABLE x I L0 L7 1
LOCALVARIABLE value I L0 L7 2
LOCALVARIABLE y I L0 L7 3
MAXSTACK = 3
MAXLOCALS = 4
The AndSC (&&) method generates two conditional jumps, as expected:
It loads value and x onto the stack, and jumps to L1 if value is lower. Else it keeps running the next lines. It loads value and y onto the stack, and jumps to L1 also, if value is greater. Else it keeps running the next lines. Which happen to be a return true in case none of the two jumps were made. And then we have the lines marked as L1 which are a return false.
The AndNonSC (&) method, however, generates three conditional jumps!
It loads value and x onto the stack and jumps to L1 if value is lower. Because now it needs to save the result to compare it with the other part of the AND, so it has to execute either "save true" or "save false", it can't do both with the same instruction. It loads value and y onto the stack and jumps to L1 if value is greater. Once again it needs to save true or false and that's two different lines depending on the comparison result. Now that both comparisons are done, the code actually executes the AND operation -- and if both are true, it jumps (for a third time) to return true; or else it continues execution onto the next line to return false.
(Preliminary) Conclusion
Though I'm not that very much experienced with Java bytecode and I may have overlooked something, it seems to me that & will actually perform worse than && in every case: it generates more instructions to execute, including more conditional jumps to predict and possibly fail at.
A rewriting of the code to replace comparisons with arithmetical operations, as someone else proposed, might be a way to make & a better option, but at the cost of making the code much less clear.
IMHO it is not worth the hassle for 99% of the scenarios (it may be very well worth it for the 1% loops that need to be extremely optimized, though).
EDIT: AMD64 assembly
As noted in the comments, the same Java bytecode can lead to different machine code in different systems, so while the Java bytecode might give us a hint about which AND version performs better, getting the actual ASM as generated by the compiler is the only way to really find out. I printed the AMD64 ASM instructions for both methods; below are the relevant lines (stripped entry points etc.).
NOTE: all methods compiled with java 1.8.0_91 unless otherwise stated.
Method AndSC with default options
# {method} {0x0000000016da0810} 'AndSC' '(III)Z' in 'AndTest'
...
0x0000000002923e3e: cmp %r8d,%r9d
0x0000000002923e41: movabs $0x16da0a08,%rax ; {metadata(method data for {method} {0x0000000016da0810} 'AndSC' '(III)Z' in 'AndTest')}
0x0000000002923e4b: movabs $0x108,%rsi
0x0000000002923e55: jl 0x0000000002923e65
0x0000000002923e5b: movabs $0x118,%rsi
0x0000000002923e65: mov (%rax,%rsi,1),%rbx
0x0000000002923e69: lea 0x1(%rbx),%rbx
0x0000000002923e6d: mov %rbx,(%rax,%rsi,1)
0x0000000002923e71: jl 0x0000000002923eb0 ;*if_icmplt
; - AndTest::AndSC@2 (line 22)
0x0000000002923e77: cmp %edi,%r9d
0x0000000002923e7a: movabs $0x16da0a08,%rax ; {metadata(method data for {method} {0x0000000016da0810} 'AndSC' '(III)Z' in 'AndTest')}
0x0000000002923e84: movabs $0x128,%rsi
0x0000000002923e8e: jg 0x0000000002923e9e
0x0000000002923e94: movabs $0x138,%rsi
0x0000000002923e9e: mov (%rax,%rsi,1),%rdi
0x0000000002923ea2: lea 0x1(%rdi),%rdi
0x0000000002923ea6: mov %rdi,(%rax,%rsi,1)
0x0000000002923eaa: jle 0x0000000002923ec1 ;*if_icmpgt
; - AndTest::AndSC@7 (line 22)
0x0000000002923eb0: mov $0x0,%eax
0x0000000002923eb5: add $0x30,%rsp
0x0000000002923eb9: pop %rbp
0x0000000002923eba: test %eax,-0x1c73dc0(%rip) # 0x0000000000cb0100
; {poll_return}
0x0000000002923ec0: retq ;*ireturn
; - AndTest::AndSC@13 (line 25)
0x0000000002923ec1: mov $0x1,%eax
0x0000000002923ec6: add $0x30,%rsp
0x0000000002923eca: pop %rbp
0x0000000002923ecb: test %eax,-0x1c73dd1(%rip) # 0x0000000000cb0100
; {poll_return}
0x0000000002923ed1: retq
Method AndSC with -XX:PrintAssemblyOptions=intel option
# {method} {0x00000000170a0810} 'AndSC' '(III)Z' in 'AndTest'
...
0x0000000002c26e2c: cmp r9d,r8d
0x0000000002c26e2f: jl 0x0000000002c26e36 ;*if_icmplt
0x0000000002c26e31: cmp r9d,edi
0x0000000002c26e34: jle 0x0000000002c26e44 ;*iconst_0
0x0000000002c26e36: xor eax,eax ;*synchronization entry
0x0000000002c26e38: add rsp,0x10
0x0000000002c26e3c: pop rbp
0x0000000002c26e3d: test DWORD PTR [rip+0xffffffffffce91bd],eax # 0x0000000002910000
0x0000000002c26e43: ret
0x0000000002c26e44: mov eax,0x1
0x0000000002c26e49: jmp 0x0000000002c26e38
Method AndNonSC with default options
# {method} {0x0000000016da0908} 'AndNonSC' '(III)Z' in 'AndTest'
...
0x0000000002923a78: cmp %r8d,%r9d
0x0000000002923a7b: mov $0x0,%eax
0x0000000002923a80: jl 0x0000000002923a8b
0x0000000002923a86: mov $0x1,%eax
0x0000000002923a8b: cmp %edi,%r9d
0x0000000002923a8e: mov $0x0,%esi
0x0000000002923a93: jg 0x0000000002923a9e
0x0000000002923a99: mov $0x1,%esi
0x0000000002923a9e: and %rsi,%rax
0x0000000002923aa1: cmp $0x0,%eax
0x0000000002923aa4: je 0x0000000002923abb ;*ifeq
; - AndTest::AndNonSC@21 (line 29)
0x0000000002923aaa: mov $0x1,%eax
0x0000000002923aaf: add $0x30,%rsp
0x0000000002923ab3: pop %rbp
0x0000000002923ab4: test %eax,-0x1c739ba(%rip) # 0x0000000000cb0100
; {poll_return}
0x0000000002923aba: retq ;*ireturn
; - AndTest::AndNonSC@25 (line 30)
0x0000000002923abb: mov $0x0,%eax
0x0000000002923ac0: add $0x30,%rsp
0x0000000002923ac4: pop %rbp
0x0000000002923ac5: test %eax,-0x1c739cb(%rip) # 0x0000000000cb0100
; {poll_return}
0x0000000002923acb: retq
Method AndNonSC with -XX:PrintAssemblyOptions=intel option
# {method} {0x00000000170a0908} 'AndNonSC' '(III)Z' in 'AndTest'
...
0x0000000002c270b5: cmp r9d,r8d
0x0000000002c270b8: jl 0x0000000002c270df ;*if_icmplt
0x0000000002c270ba: mov r8d,0x1 ;*iload_2
0x0000000002c270c0: cmp r9d,edi
0x0000000002c270c3: cmovg r11d,r10d
0x0000000002c270c7: and r8d,r11d
0x0000000002c270ca: test r8d,r8d
0x0000000002c270cd: setne al
0x0000000002c270d0: movzx eax,al
0x0000000002c270d3: add rsp,0x10
0x0000000002c270d7: pop rbp
0x0000000002c270d8: test DWORD PTR [rip+0xffffffffffce8f22],eax # 0x0000000002910000
0x0000000002c270de: ret
0x0000000002c270df: xor r8d,r8d
0x0000000002c270e2: jmp 0x0000000002c270c0
First of all, the generated ASM code differs depending on whether we choose the default AT&T syntax or the Intel syntax.
With AT&T syntax: The ASM code is actually longer for the AndSC method, with every bytecode IF_ICMP* translated to two assembly jump instructions, for a total of 4 conditional jumps. Meanwhile, for the AndNonSC method the compiler generates a more straight-forward code, where each bytecode IF_ICMP* is translated to only one assembly jump instruction, keeping the original count of 3 conditional jumps.
The ASM code is actually longer for the AndSC method, with every bytecode IF_ICMP* translated to two assembly jump instructions, for a total of 4 conditional jumps.
Meanwhile, for the AndNonSC method the compiler generates a more straight-forward code, where each bytecode IF_ICMP* is translated to only one assembly jump instruction, keeping the original count of 3 conditional jumps.
With Intel syntax: The ASM code for AndSC is shorter, with just 2 conditional jumps (not counting the non-conditional jmp at the end). Actually it's just two CMP, two JL/E and a XOR/MOV depending on the result. The ASM code for AndNonSC is now longer than the AndSC one! However, it has just 1 conditional jump (for the first comparison), using the registers to directly compare the first result with the second, without any more jumps.
The ASM code for AndSC is shorter, with just 2 conditional jumps (not counting the non-conditional jmp at the end). Actually it's just two CMP, two JL/E and a XOR/MOV depending on the result.
The ASM code for AndNonSC is now longer than the AndSC one! However, it has just 1 conditional jump (for the first comparison), using the registers to directly compare the first result with the second, without any more jumps.
Conclusion after ASM code analysis
At AMD64 machine-language level, the & operator seems to generate ASM code with fewer conditional jumps, which might be better for high prediction-failure rates (random values for example).
On the other hand, the && operator seems to generate ASM code with fewer instructions (with the -XX:PrintAssemblyOptions=intel option anyway), which might be better for really long loops with prediction-friendly inputs, where the fewer number of CPU cycles for each comparison can make a difference in the long run.
As I stated in some of the comments, this is going to vary greatly between systems, so if we're talking about branch-prediction optimization, the only real answer would be: it depends on your JVM implementation, your compiler, your CPU and your input data.
Addendum: Guava's isPowerOfTwo method
Here, Guava's developers have come up with a neat way of calculating if a given number is a power of 2:
public static boolean isPowerOfTwo(long x) {
return x > 0 & (x & (x - 1)) == 0;
}
Quoting OP:
is this use of & (where && would be more normal) a real optimization?
To find out if it is, I added two similar methods to my test class:
public boolean isPowerOfTwoAND(long x) {
return x > 0 & (x & (x - 1)) == 0;
}
public boolean isPowerOfTwoANDAND(long x) {
return x > 0 && (x & (x - 1)) == 0;
}
Intel's ASM code for Guava's version
# {method} {0x0000000017580af0} 'isPowerOfTwoAND' '(J)Z' in 'AndTest'
# this: rdx:rdx = 'AndTest'
# parm0: r8:r8 = long
...
0x0000000003103bbe: movabs rax,0x0
0x0000000003103bc8: cmp rax,r8
0x0000000003103bcb: movabs rax,0x175811f0 ; {metadata(method data for {method} {0x0000000017580af0} 'isPowerOfTwoAND' '(J)Z' in 'AndTest')}
0x0000000003103bd5: movabs rsi,0x108
0x0000000003103bdf: jge 0x0000000003103bef
0x0000000003103be5: movabs rsi,0x118
0x0000000003103bef: mov rdi,QWORD PTR [rax+rsi*1]
0x0000000003103bf3: lea rdi,[rdi+0x1]
0x0000000003103bf7: mov QWORD PTR [rax+rsi*1],rdi
0x0000000003103bfb: jge 0x0000000003103c1b ;*lcmp
0x0000000003103c01: movabs rax,0x175811f0 ; {metadata(method data for {method} {0x0000000017580af0} 'isPowerOfTwoAND' '(J)Z' in 'AndTest')}
0x0000000003103c0b: inc DWORD PTR [rax+0x128]
0x0000000003103c11: mov eax,0x1
0x0000000003103c16: jmp 0x0000000003103c20 ;*goto
0x0000000003103c1b: mov eax,0x0 ;*lload_1
0x0000000003103c20: mov rsi,r8
0x0000000003103c23: movabs r10,0x1
0x0000000003103c2d: sub rsi,r10
0x0000000003103c30: and rsi,r8
0x0000000003103c33: movabs rdi,0x0
0x0000000003103c3d: cmp rsi,rdi
0x0000000003103c40: movabs rsi,0x175811f0 ; {metadata(method data for {method} {0x0000000017580af0} 'isPowerOfTwoAND' '(J)Z' in 'AndTest')}
0x0000000003103c4a: movabs rdi,0x140
0x0000000003103c54: jne 0x0000000003103c64
0x0000000003103c5a: movabs rdi,0x150
0x0000000003103c64: mov rbx,QWORD PTR [rsi+rdi*1]
0x0000000003103c68: lea rbx,[rbx+0x1]
0x0000000003103c6c: mov QWORD PTR [rsi+rdi*1],rbx
0x0000000003103c70: jne 0x0000000003103c90 ;*lcmp
0x0000000003103c76: movabs rsi,0x175811f0 ; {metadata(method data for {method} {0x0000000017580af0} 'isPowerOfTwoAND' '(J)Z' in 'AndTest')}
0x0000000003103c80: inc DWORD PTR [rsi+0x160]
0x0000000003103c86: mov esi,0x1
0x0000000003103c8b: jmp 0x0000000003103c95 ;*goto
0x0000000003103c90: mov esi,0x0 ;*iand
0x0000000003103c95: and rsi,rax
0x0000000003103c98: and esi,0x1
0x0000000003103c9b: mov rax,rsi
0x0000000003103c9e: add rsp,0x50
0x0000000003103ca2: pop rbp
0x0000000003103ca3: test DWORD PTR [rip+0xfffffffffe44c457],eax # 0x0000000001550100
0x0000000003103ca9: ret
Intel's asm code for && version
# {method} {0x0000000017580bd0} 'isPowerOfTwoANDAND' '(J)Z' in 'AndTest'
# this: rdx:rdx = 'AndTest'
# parm0: r8:r8 = long
...
0x0000000003103438: movabs rax,0x0
0x0000000003103442: cmp rax,r8
0x0000000003103445: jge 0x0000000003103471 ;*lcmp
0x000000000310344b: mov rax,r8
0x000000000310344e: movabs r10,0x1
0x0000000003103458: sub rax,r10
0x000000000310345b: and rax,r8
0x000000000310345e: movabs rsi,0x0
0x0000000003103468: cmp rax,rsi
0x000000000310346b: je 0x000000000310347b ;*lcmp
0x0000000003103471: mov eax,0x0
0x0000000003103476: jmp 0x0000000003103480 ;*ireturn
0x000000000310347b: mov eax,0x1 ;*goto
0x0000000003103480: and eax,0x1
0x0000000003103483: add rsp,0x40
0x0000000003103487: pop rbp
0x0000000003103488: test DWORD PTR [rip+0xfffffffffe44cc72],eax # 0x0000000001550100
0x000000000310348e: ret
In this specific example, the JIT compiler generates far less assembly code for the && version than for Guava's & version (and, after yesterday's results, I was honestly surprised by this).
Compared to Guava's, the && version translates to 25% less bytecode for JIT to compile, 50% less assembly instructions, and only two conditional jumps (the & version has four of them).
So everything points to Guava's & method being less efficient than the more "natural" && version.
... Or is it?
As noted before, I'm running the above examples with Java 8:
C:\....>java -version
java version "1.8.0_91"
Java(TM) SE Runtime Environment (build 1.8.0_91-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.91-b14, mixed mode)
But what if I switch to Java 7?
C:\....>c:\jdk1.7.0_79\bin\java -version
java version "1.7.0_79"
Java(TM) SE Runtime Environment (build 1.7.0_79-b15)
Java HotSpot(TM) 64-Bit Server VM (build 24.79-b02, mixed mode)
C:\....>c:\jdk1.7.0_79\bin\java -XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=print,*AndTest.isPowerOfTwoAND -XX:PrintAssemblyOptions=intel AndTestMain
.....
0x0000000002512bac: xor r10d,r10d
0x0000000002512baf: mov r11d,0x1
0x0000000002512bb5: test r8,r8
0x0000000002512bb8: jle 0x0000000002512bde ;*ifle
0x0000000002512bba: mov eax,0x1 ;*lload_1
0x0000000002512bbf: mov r9,r8
0x0000000002512bc2: dec r9
0x0000000002512bc5: and r9,r8
0x0000000002512bc8: test r9,r9
0x0000000002512bcb: cmovne r11d,r10d
0x0000000002512bcf: and eax,r11d ;*iand
0x0000000002512bd2: add rsp,0x10
0x0000000002512bd6: pop rbp
0x0000000002512bd7: test DWORD PTR [rip+0xffffffffffc0d423],eax # 0x0000000002120000
0x0000000002512bdd: ret
0x0000000002512bde: xor eax,eax
0x0000000002512be0: jmp 0x0000000002512bbf
.....
Surprise! The assembly code generated for the & method by the JIT compiler in Java 7, has only one conditional jump now, and is way shorter! Whereas the && method (you'll have to trust me on this one, I don't want to clutter the ending!) remains about the same, with its two conditional jumps and a couple less instructions, tops.
Looks like Guava's engineers knew what they were doing, after all! (if they were trying to optimize Java 7 execution time, that is ;-)
So back to OP's latest question:
is this use of & (where && would be more normal) a real optimization?
And IMHO the answer is the same, even for this (very!) specific scenario: it depends on your JVM implementation, your compiler, your CPU and your input data.
For those kind of questions you should run a microbenchmark. I used JMH for this test.
The benchmarks are implemented as
// boolean logical AND
bh.consume(value >= x & y <= value);
and
// conditional AND
bh.consume(value >= x && y <= value);
and
// bitwise OR, as suggested by Joop Eggen
bh.consume(((value - x) | (y - value)) >= 0)
With values for value, x and y according to the benchmark name.
The result (five warmup and ten measurement iterations) for throughput benchmarking is:
Benchmark Mode Cnt Score Error Units
Benchmark.isBooleanANDBelowRange thrpt 10 386.086 ▒ 17.383 ops/us
Benchmark.isBooleanANDInRange thrpt 10 387.240 ▒ 7.657 ops/us
Benchmark.isBooleanANDOverRange thrpt 10 381.847 ▒ 15.295 ops/us
Benchmark.isBitwiseORBelowRange thrpt 10 384.877 ▒ 11.766 ops/us
Benchmark.isBitwiseORInRange thrpt 10 380.743 ▒ 15.042 ops/us
Benchmark.isBitwiseOROverRange thrpt 10 383.524 ▒ 16.911 ops/us
Benchmark.isConditionalANDBelowRange thrpt 10 385.190 ▒ 19.600 ops/us
Benchmark.isConditionalANDInRange thrpt 10 384.094 ▒ 15.417 ops/us
Benchmark.isConditionalANDOverRange thrpt 10 380.913 ▒ 5.537 ops/us
The result is not that different for the evaluation itself. As long no perfomance impact is spotted on that piece of code I would not try to optimize it. Depending on the place in the code the hotspot compiler might decide to do some optimization. Which probably is not covered by the above benchmarks.
some references:
boolean logical AND - the result value is true if both operand values are true; otherwise, the result is false
conditional AND - is like &, but evaluates its right-hand operand only if the value of its left-hand operand is true
bitwise OR - the result value is the bitwise inclusive OR of the operand values
&. So, answering the original question - no, JVM still generates a conditional branch for &.
& might be better for some cases! Comments are welcome :-)
methodWithSideEffects() will not be skipped, otherwise it would be a spec violation. However, a method without side effects could be optimized out in this case.
I'm going to come at this from a different angle.
Consider these two code fragments,
if (value >= x && value <= y) {
and
if (value >= x & value <= y) {
If we assume that value, x, y have a primitive type, then those two (partial) statements will give the same outcome for all possible input values. (If wrapper types are involved, then they are not exactly equivalent because of an implicit null test for y that might fail in the & version and not the && version.)
If the JIT compiler is doing a good job, its optimizer will be able to deduce that those two statements do the same thing:
If one is predictably faster than the other, then it should be able to use the faster version ... in the JIT compiled code.
If not, then it doesn't matter which version is used at the source code level.
Since the JIT compiler gathers path statistics before compiling, it can potentially have more information about the execution characteristics that the programmer(!).
If the current generation JIT compiler (on any given platform) doesn't optimize well enough to handle this, the next generation could well do ... depending on whether or not empirical evidence points to this being a worthwhile pattern to optimize.
Indeed, if you write you Java code in a way that optimizes for this, there is a chance that by picking the more "obscure" version of the code, you might inhibit the current or future JIT compiler's ability to optimize.
In short, I don't think you should do this kind of micro-optimization at the source code level. And if you accept this argument1, and follow it to its logical conclusion, the question of which version is faster is ... moot2.
1 - I do not claim this is anywhere near being a proof.
2 - Unless you are one of the tiny community of people who actually write Java JIT compilers ...
The "Very Famous Question" is interesting in two respects:
On the one hand, that is an example where the kind of optimization required to make a difference is way beyond the capability of a JIT compiler.
On the other hand, it would not necessarily be the correct thing to sort the array ... just because a sorted array can be processed faster. The cost of sorting the array, could well be (much) greater than the saving.
x and y are either constants or predictable value, the optimized code will rather look like value-x ≤ͧ y-x where ≤ͧ is an unsigned long comparison and y-x a constant, though even if x and y are not predictable, that single comparison variant might be used, if two branches are considered more expensive than an eagerly performed comparison (a numeric comparison is on par with the minus operation). So thinking about & and && makes indeed no sense.
Using either & or && still requires a condition to be evaluated so it's unlikely it will save any processing time - it might even add to it considering you're evaluating both expressions when you only need to evaluate one.
Using & over && to save a nanosecond if that in some very rare situations is pointless, you've already wasted more time contemplating the difference than you would've saved using & over &&.
Edit
I got curious and decided to run some bench marks.
I made this class:
public class Main {
static int x = 22, y = 48;
public static void main(String[] args) {
runWithOneAnd(30);
runWithTwoAnds(30);
}
static void runWithOneAnd(int value){
if(value >= x & value <= y){
}
}
static void runWithTwoAnds(int value){
if(value >= x && value <= y){
}
}
}
and ran some profiling tests with NetBeans. I didn't use any print statements to save processing time, just know both evaluate to true.
First test:
https://i.stack.imgur.com/1e8iG.png
Second test:
https://i.stack.imgur.com/bxZ7t.png
Third test:
https://i.stack.imgur.com/W0L6z.png
As you can see by the profiling tests, using only one & actually takes 2-3 times longer to run compared to using two &&. This does strike as some what odd as i did expect better performance from only one &.
I'm not 100% sure why. In both cases, both expressions have to be evaluated because both are true. I suspect that the JVM does some special optimization behind the scenes to speed it up.
Moral of the story: convention is good and premature optimization is bad.
Edit 2
I redid the benchmark code with @SvetlinZarev's comments in mind and a few other improvements. Here is the modified benchmark code:
public class Main {
static int x = 22, y = 48;
public static void main(String[] args) {
oneAndBothTrue();
oneAndOneTrue();
oneAndBothFalse();
twoAndsBothTrue();
twoAndsOneTrue();
twoAndsBothFalse();
System.out.println(b);
}
static void oneAndBothTrue() {
int value = 30;
for (int i = 0; i < 2000; i++) {
if (value >= x & value <= y) {
doSomething();
}
}
}
static void oneAndOneTrue() {
int value = 60;
for (int i = 0; i < 4000; i++) {
if (value >= x & value <= y) {
doSomething();
}
}
}
static void oneAndBothFalse() {
int value = 100;
for (int i = 0; i < 4000; i++) {
if (value >= x & value <= y) {
doSomething();
}
}
}
static void twoAndsBothTrue() {
int value = 30;
for (int i = 0; i < 4000; i++) {
if (value >= x & value <= y) {
doSomething();
}
}
}
static void twoAndsOneTrue() {
int value = 60;
for (int i = 0; i < 4000; i++) {
if (value >= x & value <= y) {
doSomething();
}
}
}
static void twoAndsBothFalse() {
int value = 100;
for (int i = 0; i < 4000; i++) {
if (value >= x & value <= y) {
doSomething();
}
}
}
//I wanted to avoid print statements here as they can
//affect the benchmark results.
static StringBuilder b = new StringBuilder();
static int times = 0;
static void doSomething(){
times++;
b.append("I have run ").append(times).append(" times \n");
}
}
And here are the performance tests:
Test 1:
https://i.stack.imgur.com/yXjrp.png
Test 2:
https://i.stack.imgur.com/HNfC8.png
Test 3:
https://i.stack.imgur.com/KUC1x.png
This takes into account different values and different conditions as well.
Using one & takes more time to run when both conditions are true, about 60% or 2 milliseconds more time. When either one or both conditions are false, then one & runs faster, but it only runs about 0.30-0.50 milliseconds faster. So & will run faster than && in most circumstances, but the performance difference is still negligible.
What you are after is something like this:
x <= value & value <= y
value - x >= 0 & y - value >= 0
((value - x) | (y - value)) >= 0 // integer bit-or
Interesting, one would almost like to look at the byte code. But hard to say. I wish this were a C question.
I was curious to the answer as well, so I wrote the following (simple) test for this:
private static final int max = 80000;
private static final int size = 100000;
private static final int x = 1500;
private static final int y = 15000;
private Random random;
@Before
public void setUp() {
this.random = new Random();
}
@After
public void tearDown() {
random = null;
}
@Test
public void testSingleOperand() {
int counter = 0;
int[] numbers = new int[size];
for (int j = 0; j < size; j++) {
numbers[j] = random.nextInt(max);
}
long start = System.nanoTime(); //start measuring after an array has been filled
for (int i = 0; i < numbers.length; i++) {
if (numbers[i] >= x & numbers[i] <= y) {
counter++;
}
}
long end = System.nanoTime();
System.out.println("Duration of single operand: " + (end - start));
}
@Test
public void testDoubleOperand() {
int counter = 0;
int[] numbers = new int[size];
for (int j = 0; j < size; j++) {
numbers[j] = random.nextInt(max);
}
long start = System.nanoTime(); //start measuring after an array has been filled
for (int i = 0; i < numbers.length; i++) {
if (numbers[i] >= x & numbers[i] <= y) {
counter++;
}
}
long end = System.nanoTime();
System.out.println("Duration of double operand: " + (end - start));
}
With the end result being that the comparison with && always wins in terms of speed, being about 1.5/2 milliseconds quicker than &.
EDIT: As @SvetlinZarev pointed out, I was also measuring the time it took Random to get an integer. Changed it to use a pre-filled array of random numbers, which caused the duration of the single operand test to wildly fluctuate; the differences between several runs were up to 6-7ms.
generated >= x), which means the predictor will usually get things right (if it works the way I think it does). I'm going to try fiddling with those 'x' and 'y' values - I think x=40000 and y=60000 will be interesting (50% success on each test).
random.nextInt() as it take s much mor etime than the simple && or &. Your tests is flawed
The way this was explained to me, is that && will return false if the first check in a series is false, while & checks all items in a series regardless of how many are false. I.E.
if (x>0 && x <=10 && x
Will run faster than
if (x>0 & x <=10 & x
If x is greater than 10, because single ampersands will continue to check the rest of the conditions whereas double ampersands will break after the first non-true condition.
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javacmight output different code than official Oracle or than the OpenJDK one... And of course the machine code in an X86 machine would probably differ from a PowerPC AIX system or the Snapdragon CPUs used in many smartphones -- every platform will have its own compilers and optimizations. But in a simple case like this, I doubt that the differences from one CPU to another will make a bigger difference than having 2 vs 3 bytecode conditional jumps.