[LLVMdev] Missed optimization on array initialization

[Duncan Sands]

Hi Carlo, for what it's worth, gcc-4.7 doesn't get this either.

Ciao, Duncan.

On 25/02/12 12:17, Carlo Alberto Ferraris wrote:
> Prompted by a SO post
> (http://stackoverflow.com/questions/9441882/compiler-instruction-reordering-optimizations-in-c-and-what-inhibits-them/9442363)
> I checked and found that LLVM yields the same (seemingly) suboptimal code as MSVC.
> Consider the following, simplified, C snippet:
>
> extern void bar(int*);
>
> void foo(int a)
> {
>      int ar[100] = {a};
>      if (a)
>          return;
>      bar(ar);
> }
>
> Ideally, the array initialization should be sank after the return, but in
> Clang/LLVM 3.0 this doesn't happen:
>
> ; ModuleID ='/tmp/webcompile/_11079_0.bc'
> target  datalayout ="e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> target  triple ="x86_64-unknown-linux-gnu"
>
> define  void  @_Z3fooi(i32  %a) uwtable {
>    %ar =alloca  [100 xi32],align  16
>    %1 =bitcast  [100 xi32]* %arto  i8*
>    call  void  @llvm.memset.p0i8.i64(i8* %1,i8  0,i64  400,i32  16,i1  false)
>    %2 =getelementptr  inbounds [100 xi32]* %ar,i64  0,i64  0
>    store  i32  %a,i32* %2,align  16, !tbaa !0
>    %3 =icmp  eq  i32  %a, 0
>    br  i1  %3,label  %4,label  %5
>
> ;<label>:4                                       ; preds = %0
>    call  void  @_Z3barPi(i32* %2)
>    br  label  %5
>
> ;<label>:5                                       ; preds = %4, %0
>    ret  void
> }
>
> declare  void  @llvm.memset.p0i8.i64(i8*nocapture,i8,i64,i32,i1)nounwind
>
> declare  void  @_Z3barPi(i32*)
>
> !0 = metadata !{metadata !"int", metadata !1}
> !1 = metadata !{metadata !"omnipotent char", metadata !2}
> !2 = metadata !{metadata !"Simple C/C++ TBAA",null}
>
> and this gets emitted as (for x64, but x86 is similar):
>
> # BB#0:
> 	pushq	%rbx
> .Ltmp3:
> 	.cfi_def_cfa_offset 16
> 	subq	$400, %rsp              # imm = 0x190
> .Ltmp4:
> 	.cfi_def_cfa_offset 416
> .Ltmp5:
> 	.cfi_offset %rbx, -16
> 	movl	%edi, %ebx
> 	leaq	(%rsp), %rdi
> 	xorl	%esi, %esi
> 	movl	$400, %edx              # imm = 0x190
> 	callq	memset
> 	movl	%ebx, (%rsp)
> 	testl	%ebx, %ebx
> 	jne	.LBB0_2
> # BB#1:
> 	leaq	(%rsp), %rdi
> 	callq	_Z3barPi
> .LBB0_2:
> 	addq	$400, %rsp              # imm = 0x190
> 	popq	%rbx
> 	ret
>
> I don't have ToT at hand, so I don't know if this is still the case. Any idea
> why this might be happening?
>
>
>
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