[X86] Fix AvoidStoreForwardingBlocks pass for negative displacements

Fixes https://bugs.llvm.org/show_bug.cgi?id=39926.

The size of the first copy was computed as
std::abs(std::abs(LdDisp2) - std::abs(LdDisp1)), which results in
skipped bytes if the signs of LdDisp2 and LdDisp1 differ. As far as
I can see, this should just be LdDisp2 - LdDisp1. The case where
LdDisp1 > LdDisp2 is already handled in the code above, in which case
LdDisp2 is set to LdDisp1 and this subtraction will evaluate to
Size1 = 0, which is the correct value to skip an overlapping copy.

Differential Revision: https://reviews.llvm.org/D55485

llvm-svn: 348750

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp b/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp
index eb9c4b3e597..2850baf7a65 100644
--- a/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp
+++ b/llvm/lib/Target/X86/X86AvoidStoreForwardingBlocks.cpp
@@ -586,7 +586,7 @@ void X86AvoidSFBPass::breakBlockedCopies(
       StDisp2 += OverlapDelta;
       Size2 -= OverlapDelta;
     }
-    Size1 = std::abs(std::abs(LdDisp2) - std::abs(LdDisp1));
+    Size1 = LdDisp2 - LdDisp1;
 
     // Build a copy for the point until the current blocking store's
     // displacement.