[llvm-commits] [llvm] r151467 - in /llvm/trunk: lib/Analysis/InstructionSimplify.cpp test/Transforms/InstCombine/icmp.ll test/Transforms/InstSimplify/compare.ll

[Nick Lewycky]

Eli Friedman wrote:
> On Tue, Feb 28, 2012 at 11:49 PM, Nick Lewycky<nicholas at mxc.ca>  wrote:
>> Eli Friedman wrote:
>>>
>>> On Sat, Feb 25, 2012 at 6:09 PM, Nick Lewycky<nicholas at mxc.ca>    wrote:
>>>>
>>>> Author: nicholas
>>>> Date: Sat Feb 25 20:09:49 2012
>>>> New Revision: 151467
>>>>
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=151467&view=rev
>>>> Log:
>>>> Reinstate the optimization from r151449 with a fix to not turn 'gep %x'
>>>> into
>>>> 'gep null' when the icmp predicate is unsigned (or is signed without
>>>> inbounds).
>>>>
>>>> Modified:
>>>>     llvm/trunk/lib/Analysis/InstructionSimplify.cpp
>>>>     llvm/trunk/test/Transforms/InstCombine/icmp.ll
>>>>     llvm/trunk/test/Transforms/InstSimplify/compare.ll
>>>
>>>
>>> This change is causing an execution failure in the gcc testsuite on
>>> gcc.c-torture/execute/frame-address.c .  Please take a look.
>>
>>
>> int check_fa_work (const char *c, const char *f)
>> {
>>   const char d = 0;
>>
>>   if (c>=&d)
>>     return c>= f&&  f>=&d;
>>   else
>>     return c<= f&&  f<=&d;
>> }
>>
>> With this patch we decide that the incoming arguments can't legitimately be
>> compared with the local variable, taking advantage of the undefined
>> behaviour. This function is optimized into "ret i32 0".
>
> The LangRef rules for icmp seem to state that the result is always
> well-defined... if we're following different rules, please update it.

Very well, I've committed new text in r151710. I think my commit log is 
a better explanation than the text in the LangRef, except that the 
former uses terms not defined in the langref. Feel free to iterate on it!

Nick