Juneyoung Lee 49bbd5d17a [DivRemPairs] Freeze operands if they can be undef values ... Summary: DivRemPairs is unsound with respect to undef values. ``` // bb1: // %rem = srem %x, %y // bb2: // %div = sdiv %x, %y // --> // bb1: // %div = sdiv %x, %y // %mul = mul %div, %y // %rem = sub %x, %mul ``` If X can be undef, X should be frozen first. For example, let's assume that Y = 1 & X = undef: ``` %div = sdiv undef, 1 // %div = undef %rem = srem undef, 1 // %rem = 0 => %div = sdiv undef, 1 // %div = undef %mul = mul %div, 1 // %mul = undef %rem = sub %x, %mul // %rem = undef - undef = undef ``` http://volta.cs.utah.edu:8080/z/m7Xrx5 Same for Y. If X = 1 and Y = (undef | 1), %rem in src is either 1 or 0, but %rem in tgt can be one of many integer values. This resolves https://bugs.llvm.org/show_bug.cgi?id=42619 . This miscompilation disappears if undef value is removed, but it may take a while. DivRemPair happens pretty late during the optimization pipeline, so this optimization seemed as a good candidate to fix without major regression using freeze than other broken optimizations. Reviewers: spatel, lebedev.ri, george.burgess.iv Reviewed By: spatel Subscribers: wuzish, regehr, nlopes, nemanjai, hiraditya, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D76483		2020-03-25 03:46:14 +09:00
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PowerPC	[DivRemPairs] Freeze operands if they can be undef values	2020-03-25 03:46:14 +09:00
X86	[DivRemPairs] Freeze operands if they can be undef values	2020-03-25 03:46:14 +09:00