[LLVMdev] LLVM Optmizer

[John Regehr]

The code here is invalid, it executes an operation with undefined behavior 
in the else branch of the first if (read from uninitialized vary).

John



On Tue, 6 Jan 2009, Manoel Teixeira wrote:

>
> The following C code :
> #include <stdio.h>
> #include <stdlib.h>
>
> int  TESTE2( int  parami , int paraml ,double  paramd  )
> {
>  int varx=0,vary;
>  int nI =0;
>  //varx= parami;
>    if( parami > 0  )
> 	{
>   varx = parami;
>   vary = varx + 1;
> 	}
>  else
>  {
>   varx = vary + 1;
>   vary = paraml;
>  }
> varx = varx + parami + paraml;
>  for( nI = 1 ;   nI <=  paraml; nI++)
>  {
>    varx =  varx + parami + 1 ;
> 	vary =  varx + nI;
>  }
>  vary = varx + 5;
>  varx = vary + paraml;
>
> return varx ;
> }
>
> Generates the IR :
> ; ModuleID = '/tmp/webcompile/_9908_0.bc'
> target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:32:32"
> target triple = "i386-pc-linux-gnu"
>
> define i32 @TESTE2(i32 %parami, i32 %paraml, double %paramd) nounwind readnone {
> entry:
> 	%0 = shl i32 %parami, 1		; <i32> [#uses=1]
> 	%varx.110 = add i32 %0, %paraml		; <i32> [#uses=2]
> 	%1 = icmp slt i32 %paraml, 1		; <i1> [#uses=1]
> 	br i1 %1, label %bb5, label %bb3
>
> bb3:		; preds = %bb3, %entry
> 	%indvar = phi i32 [ %indvar.next, %bb3 ], [ 0, %entry ]		; <i32> [#uses=3]
> 	%2 = add i32 %indvar, 2		; <i32> [#uses=1]
> 	%3 = icmp sgt i32 %2, %paraml		; <i1> [#uses=1]
> 	%indvar.next = add i32 %indvar, 1		; <i32> [#uses=1]
> 	br i1 %3, label %bb5.loopexit, label %bb3
>
> bb5.loopexit:		; preds = %bb3
> 	%tmp14 = add i32 %parami, 1		; <i32> [#uses=1]
> 	%tmp15 = mul i32 %indvar, %tmp14		; <i32> [#uses=1]
> 	%varx.111 = add i32 %tmp15, %varx.110		; <i32> [#uses=1]
> 	%4 = add i32 %varx.111, %parami		; <i32> [#uses=1]
> 	%phitmp = add i32 %4, 1		; <i32> [#uses=1]
> 	br label %bb5
>
> bb5:		; preds = %bb5.loopexit, %entry
> 	%varx.1.lcssa = phi i32 [ %varx.110, %entry ], [ %phitmp, %bb5.loopexit ]		; <i32> [#uses=1]
> 	%5 = add i32 %paraml, 5		; <i32> [#uses=1]
> 	%6 = add i32 %5, %varx.1.lcssa		; <i32> [#uses=1]
> 	ret i32 %6
> }
>
>
> While the MSVC generates the assemble :
> PUBLIC	_TESTE
> ; Function compile flags: /Ogtpy
> ; File c:\msys\1.0\home\mteixeira\testeadvpl.c
> ;	COMDAT _TESTE
> _TEXT	SEGMENT
> _parami$ = 8						; size = 4
> _paraml$ = 12						; size = 4
> _paramd$ = 16						; size = 8
> _TESTE	PROC						; COMDAT
>
> ; 6    :   int varx=0,vary;
> ; 7    :   int nI =0;
> ; 8    :   //varx= parami;
> ; 9    :     if( parami > 0  )
>
> 	mov	ecx, DWORD PTR _parami$[esp-4]
>
> ; 10   : 	{
> ; 11   :    varx = parami;
> ; 12   :    vary = 0;
> ; 13   : 	}
> ; 14   :   else
> ; 15   :   {
> ; 16   :    varx = 0;
> ; 17   :    vary = paraml;
> ; 18   :   }
> ; 19   :   for( nI = 1 ;   nI <=  paraml; nI++)
>
> 	mov	edx, DWORD PTR _paraml$[esp-4]
> 	xor	eax, eax
> 	test	ecx, ecx
> 	setle	al
> 	sub	eax, 1
> 	and	eax, ecx
> 	cmp	edx, 1
> 	jl	SHORT $LN3 at TESTE
> 	add	ecx, 1
> 	imul	ecx, edx
> 	add	eax, ecx
> $LN3 at TESTE:
>
> ; 20   :   {
> ; 21   :     varx =  varx + parami + 1 ;
> ; 22   : 	vary =  varx + nI;
> ; 23   :   }
> ; 24   :
> ; 25   : return varx ;
> ; 26   : }
>
> 	ret	0
> _TESTE	ENDP
> _TEXT	ENDS
>
> Running the same code, the objetc generated with MSVC is 600 times faster than that generate with the LLVM compiler
> Is threre any way to get the same optimzation with the LLVM?
>
>
> Manoel Teixeira
>
>
>
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