[llvm-bugs] [Bug 31449] New: Miscompile of spec2006/perlbench

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Dec 21 10:55:59 PST 2016


            Bug ID: 31449
           Summary: Miscompile of spec2006/perlbench
           Product: new-bugs
           Version: trunk
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: mssimpso at codeaurora.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Top-of-trunk is currently miscompiling spec2006/perlbench on AArch64 for "-O1"
and above. For example, with "clang -target=aarch64-linux-none-gnu -O2 ...".
I've bisected the miscompile to:

commit 7b626dfd628404afb5fb410e8cb9eedf8e1722b4
Author: Ehsan Amiri <amehsan at ca.ibm.com>
Date:   Thu Dec 15 12:25:13 2016 +0000

[InstCombine] New opportunities for FoldAndOfICmp and FoldXorOfICmp

A number of new patterns for simplifying and/xor of icmp:

(icmp ne %x, 0) ^ (icmp ne %y, 0) => icmp ne %x, %y if the following is true:
1- (%x = and %a, %mask) and (%y = and %b, %mask)
2- %mask is a power of 2.

(icmp eq %x, 0) & (icmp ne %y, 0) => icmp ult %x, %y if the following is true:
1- (%x = and %a, %mask1) and (%y = and %b, %mask2)
2- Let %t be the smallest power of 2 where %mask1 & %t != 0. Then for any 
   %s that is a power of 2 and %s & %mask2 != 0, we must have %s <= %t. 
For example if %mask1 = 24 and %mask2 = 16, setting %s = 16 and %t = 8 
violates condition (2) above. So this optimization cannot be applied.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@289813

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