[llvm-commits] [llvm] r61297 - in /llvm/trunk: lib/Transforms/Scalar/SimplifyLibCalls.cpp test/Transforms/SimplifyLibCalls/2008-12-20-StrcmpMemcmp.ll

[Nick Lewycky]

Eli Friedman wrote:
> On Sat, Dec 20, 2008 at 4:38 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>> On Sat, Dec 20, 2008 at 4:19 PM, Nick Lewycky <nicholas at mxc.ca> wrote:
>>> Author: nicholas
>>> Date: Sat Dec 20 18:19:21 2008
>>> New Revision: 61297
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=61297&view=rev
>>> Log:
>>> Turn strcmp into memcmp, such as strcmp(P, "x") --> memcmp(P, "x", 2).
>> I'm pretty sure this isn't safe; take the following testcase:
>>
>> int foo(char* x) {return strcmp(x, "x", 2) == 0;}
> 
> Oops, messed that up slightly; try the following:
> int foo(char* x) {return strcmp(x, "x") == 0;}

The pathological case is:

   char x[1] = "\0";  // pretend we don't know its length
   char y[2] = "x\0"; // we know its length
   return strcmp(x, y);

If we transform it into

   return memcmp(x, y, 2)

then the existing MemCmpOpt will exercise these cases:

   // memcmp(S1,S2,2) != 0 -> (*(short*)LHS ^ *(short*)RHS)  != 0
   // memcmp(S1,S2,4) != 0 -> (*(int*)LHS ^ *(int*)RHS)  != 0

resulting in a 2-byte load of x[1], which is illegal.

Could someone comment on what part of this transform is wrong? I recall 
having a discussion about it on IRC, but can't recall the details...

Nick