[PATCH] Fix crash in loop idiom recognizer

[Hal Finkel]

LGTM.

 -Hal

----- Original Message -----
> From: "Pete Cooper" <peter_cooper at apple.com>
> To: "llvm-commits" <llvm-commits at cs.uiuc.edu>
> Sent: Wednesday, July 8, 2015 1:35:38 PM
> Subject: [PATCH] Fix crash in loop idiom recognizer
> 
> 
> 
> Hi all
> 
> When spotting that a loop can use ctpop, we were incorrectly
> replacing all uses of a value with a value derived from ctpop.
> 
> The bug here was exposed because we were replacing a use prior to the
> ctpop with the ctpop value and so we have a use before def, i.e., we
> changed
> 
> %tobool.5 = icmp ne i32 %num, 0
> store i1 %tobool.5, i1* %ptr
> br i1 %tobool.5, label %for.body.lr.ph, label %for.end
> 
> to
> 
> store i1 %1, i1* %ptr
> %0 = call i32 @llvm.ctpop.i32(i32 %num)
> %1 = icmp ne i32 %0, 0
> br i1 %1, label %for.body.lr.ph, label %for.end
> 
> Even if we inserted the ctpop so that it dominates the store here,
> that would still be incorrect. The store doesn’t want the result of
> ctpop.
> 
> The fix is very simple, and involves replacing only the branch
> condition with the ctpop instead of all uses.
> 
> Cheers,
> Pete
> 
> 
> 
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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory