[LLVMdev] GCC vs. LLVM difference on simple code example

[Chris Lattner]

On Mar 24, 2011, at 11:59 AM, Marc de Kruijf wrote:

> Hi,
> 
> I have a question on why gcc and llvm-gcc compile the following simple code snippet differently:
> 
> extern int a;
> extern int *b;
> 
> void foo() {
>   int i;
>   for (i = 1; i < 100; ++i) 
>     a += b[i];
> }
> 
> gcc compiles this function hoisting the load of the global variable "b" outside of the loop, while llvm-gcc keeps it inside the loop.  This results in slower code on the part of llvm-gcc, and I'm wondering why this choice is made?  Is it because of the memory consistency model?  With respect to memory consistency, does the C standard say whether a global variable used inside a function is loaded at the point of the use(s), or whether it can be loaded by the compiler earlier in the function?  I had always thought that it was legal to hoist the load of a global variable outside of the loop as long as it was not declared volatile....

The difference here is that llvm-gcc doesn't support the "-fstrict-aliasing" flag.  If you pass  -fno-strict-aliasing to gcc, you'll probably get similar code to llvm-gcc.  Note that clang does support -fstrict-aliasing.

-Chris

> 
> Here is the x86 assembly code generated by gcc 4.5.2.  The load of "b" is highlighted:
> 
>         .file   "foo.c"
>         .text
>         .p2align 4,,15
> .globl foo
>         .type   foo, @function
> foo:
>         movl    b, %ecx
>         movl    $1, %eax
>         movl    a, %edx
>         pushl   %ebp
>         movl    %esp, %ebp
>         .p2align 4,,7
>         .p2align 3
> .L2:
>         addl    (%ecx,%eax,4), %edx
>         addl    $1, %eax
>         cmpl    $100, %eax
>         movl    %edx, a
>         jne     .L2
>         popl    %ebp
>         ret
>         .size   foo, .-foo
>         .ident  "GCC: (GNU) 4.5.2"
>         .section        .note.GNU-stack,"", at progbits
> 
> And here is the code produced by llvm-gcc 4.2.1:
> 
>         .file   "foo.c"
>         .text
>         .globl  foo
>         .align  16, 0x90
>         .type   foo, at function
> foo:
>         pushl   %ebp
>         movl    %esp, %ebp
>         movl    $1, %eax
>         movl    a, %ecx
>         .align  16, 0x90
> .LBB0_1:
>         movl    b, %edx
>         addl    (%edx,%eax,4), %ecx
>         movl    %ecx, a
>         incl    %eax
>         cmpl    $100, %eax
>         jne     .LBB0_1
>         popl    %ebp
>         ret
> .Ltmp0:
>         .size   foo, .Ltmp0-foo
>         .section        .note.GNU-stack,"", at progbits
>         .ident  "GCC: (GNU) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build)"
> 
> 
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