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

[Marc de Kruijf]

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....

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)"
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20110324/7d2108ef/attachment.html>