[LLVMbugs] Rewriting of strcmp bug and patch

[Chris Lattner]

On Thu, 16 Jun 2005, John Mellor-Crummey wrote:

> strcmp of a string with null was being optimized away incorrectly. Rather 
> than testing the first character
> of the string, it was attempting to test the first character of the null.
>
> In the example we were looking at, it seemed that we also were running into a 
> similar problem with a missing cast (which we previously encountered with a 
> bug rewriting sprintf); we applied the safe (cast if necessary) operation 
> developed by Chris. Use of this static function in a new place required 
> either a forward declaration of the function or moving the definition of the 
> function. We moved the definition. You might prefer the forward declaration.
>
> In any case, the patch is below.

This patch looks great, thanks!  One request: can you attach the patch to 
an email, instead of including it inline?  It appears that your mailer is 
wrapping lines and removing spaces that prevents the patch from applying.

Thanks again,

-Chris

> John Mellor-Crummey
>
>
> cvs diff -C3 SimplifyLibCalls.cpp
> cvs diff: warning: failed to open /home/johnmc/.cvspass for reading: No such 
> file or directory
> Index: SimplifyLibCalls.cpp
> ===================================================================
> RCS file: /var/cvs/llvm/llvm/lib/Transforms/IPO/SimplifyLibCalls.cpp,v
> retrieving revision 1.42
> diff -C3 -r1.42 SimplifyLibCalls.cpp
> *** SimplifyLibCalls.cpp        21 May 2005 01:27:04 -0000      1.42
> --- SimplifyLibCalls.cpp        16 Jun 2005 12:42:06 -0000
> ***************
> *** 619,624 ****
> --- 619,636 ----
>    }
>  } StrChrOptimizer;
>
> +
> + /// CastToCStr - Return V if it is an sbyte*, otherwise cast it to sbyte*,
> + /// inserting the cast before IP, and return the cast.
> + /// @brief Cast a value to a "C" string.
> + static Value *CastToCStr(Value *V, Instruction &IP) {
> +   const Type *SBPTy = PointerType::get(Type::SByteTy);
> +   if (V->getType() != SBPTy)
> +     return new CastInst(V, SBPTy, V->getName(), &IP);
> +   return V;
> + }
> +
> +
>  /// This LibCallOptimization will simplify a call to the strcmp library
>  /// function.  It optimizes out cases where one or both arguments are 
> constant
>  /// and the result can be determined statically.
> ***************
> *** 663,669 ****
>        if (len_1 == 0)
>        {
>          // strcmp("",x) -> *x
> !         LoadInst* load = new LoadInst(s1,ci->getName()+".load",ci);
>          CastInst* cast =
>            new CastInst(load,Type::IntTy,ci->getName()+".int",ci);
>          ci->replaceAllUsesWith(cast);
> --- 675,681 ----
>        if (len_1 == 0)
>        {
>          // strcmp("",x) -> *x
> !         LoadInst* load = new 
> LoadInst(CastToCStr(s2,*ci),ci->getName()+".load",ci);
>          CastInst* cast =
>            new CastInst(load,Type::IntTy,ci->getName()+".int",ci);
>          ci->replaceAllUsesWith(cast);
> ***************
> *** 681,687 ****
>        if (len_2 == 0)
>        {
>          // strcmp(x,"") -> *x
> !         LoadInst* load = new LoadInst(s2,ci->getName()+".val",ci);
>          CastInst* cast =
>            new CastInst(load,Type::IntTy,ci->getName()+".int",ci);
>          ci->replaceAllUsesWith(cast);
> --- 693,699 ----
>        if (len_2 == 0)
>        {
>          // strcmp(x,"") -> *x
> !         LoadInst* load = new 
> LoadInst(CastToCStr(s1,*ci),ci->getName()+".val",ci);
>          CastInst* cast =
>            new CastInst(load,Type::IntTy,ci->getName()+".int",ci);
>          ci->replaceAllUsesWith(cast);
> ***************
> *** 1242,1256 ****
>    }
>  } PowOptimizer;
>
> - /// CastToCStr - Return V if it is an sbyte*, otherwise cast it to sbyte*,
> - /// inserting the cast before IP, and return the cast.
> - /// @brief Cast a value to a "C" string.
> - static Value *CastToCStr(Value *V, Instruction &IP) {
> -   const Type *SBPTy = PointerType::get(Type::SByteTy);
> -   if (V->getType() != SBPTy)
> -     return new CastInst(V, SBPTy, V->getName(), &IP);
> -   return V;
> - }
>
>  /// This LibCallOptimization will simplify calls to the "fprintf" library
>  /// function. It looks for cases where the result of fprintf is not used 
> and the
> --- 1254,1259 ----
>
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>

-Chris

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