Andrea Di Biagio a34cab9be7 [InstCombine] Fix wrong folding of constant comparison involving ahsr and negative quantities (PR20945). ... Example: define i1 @foo(i32 %a) { %shr = ashr i32 -9, %a %cmp = icmp ne i32 %shr, -5 ret i1 %cmp } Before this fix, the instruction combiner wrongly thought that %shr could have never been equal to -5. Therefore, %cmp was always folded to 'true'. However, when %a is equal to 1, then %cmp evaluates to 'false'. Therefore, in this example, it is not valid to fold %cmp to 'true'. The problem was only affecting the case where the comparison was between negative quantities where one of the quantities was obtained from arithmetic shift of a negative constant. This patch fixes the problem with the wrong folding (fixes PR20945). With this patch, the 'icmp' from the example is now simplified to a comparison between %a and 1. This still allows us to get rid of the arithmetic shift (%shr). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217950 91177308-0d34-0410-b5e6-96231b3b80d8		2014-09-17 11:32:31 +00:00
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Transforms	[InstCombine] Fix wrong folding of constant comparison involving ahsr and negative quantities (PR20945).	2014-09-17 11:32:31 +00:00
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