[PATCH] D101970: [X86FixupLEAs] Transform the sequence LEA/SUB to SUB/SUB

[David Zarzycki via Phabricator via llvm-commits]

davezarzycki added a comment.

I can check it, but I'm a bit concerned about the comment and what it implies. The problem is NOT overflow, but that comparison operations need to be redistributed during this optimization (or dependent comparisons need to disable the optimization). So in pseudocode, the original code that miscompiled looks like this:

if (x - (y + z) <= 0) goto something; // where <= is a unsigned comparison but it really doesn't matter.

So redistributing the arithmetic requires redistributing the comparison.

auto t1 = x - y
if (t1 <= 0) goto something;
if (t1 - z <= 0) goto something;

Does the new patch redistribute comparisons?


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  https://reviews.llvm.org/D101970/new/

https://reviews.llvm.org/D101970