[LLVMdev] "equivalent" .ll files diverge after optimizations are applied

Chris Lattner clattner at apple.com
Tue Aug 31 22:19:17 PDT 2010

On Aug 31, 2010, at 11:18 AM, Argyrios Kyrtzidis wrote:

> Hi,
> I've attached 2 .ll files which are supposed to be equivalent but 'unopt-fail.ll' causes a crash in webkit's test suite while 'unopt-pass.ll' does not. I can't give more details about the crash, when I run the crashing test it in isolation it passes, when I run the full suite it crashes; it boggles the mind.
> Below I provide the optimized asm that is produced from each file. Could you give a hint on what is the problem ?
> I also attached 't.cpp' which approximates the source that the .ll files came from.

Hi Argiris,

The real problem here is that the X86 backend is turning datatypes like <1 x i64> into MMX operations, but doesn't do so in a safe way (it's not inserting the requisite EMMS instructions).  After discussing this with Dale and Bill, the right fix is to stop mapping generic vectors onto MMX operations.  This will define away the existing -disable-mmx flag and make stuff like this impossible.

However, this isn't going to happen in the next couple days, certainly not in time for the 2.8 release branch on friday.  As such, I checked in a horrible hack in r112696 that prevents SRoA from introducing mmx specific vector types.  I'm not aware of a target where those datatypes are actually useful, so this shouldn't be bad.  On your testcase, no mmx operations are produced.

Please let me know if you see any other MMX stuff being generated.


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