[llvm-bugs] [Bug 43818] New: UBSan error negating INT64_MIN in AArch64TargetLowering::LowerSELECT_CC

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43818

            Bug ID: 43818
           Summary: UBSan error negating INT64_MIN in
                    AArch64TargetLowering::LowerSELECT_CC
           Product: libraries
           Version: 9.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: AArch64
          Assignee: unassignedbugs at nondot.org
          Reporter: andrew at scheidecker.net
                CC: arnaud.degrandmaison at arm.com,
                    llvm-bugs at lists.llvm.org, peter.smith at linaro.org,
                    Ties.Stuij at arm.com

In a LLVM 9 build with -DLLVM_USE_SANITIZER=Undefined, run:

  llc -march=aarch64 repro.ll

where repro.ll contains:

  define i64 @0(i1) {
  entry:
    %1 = select i1 %0, i64 0, i64 -9223372036854775808
    ret i64 %1
  }

This produces the error:

  <snip>/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:4867:29: runtime
error: negation of -9223372036854775808 cannot be represented in type 'int64_t'
(aka 'long'); cast to an unsigned type to negate this value to itself
  SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
<snip>/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:4867:29 in

It looks like the bug is still present in the tip of tree code, though I
haven't tested it. Here's the code in question:

 
https://github.com/llvm/llvm-project/blob/073ab70b72fa0db7d9c591f77377ef3ce88cc222/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp#L5076

  const int64_t TrueVal = CTVal->getSExtValue();
  const int64_t FalseVal = CFVal->getSExtValue();
  //<snip>
  } else if (TrueVal == -FalseVal) {
                        ^~~~~~~~~

FalseVal is INT64_MIN, and negating it is undefined behavior.

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