[llvm-bugs] [Bug 34133] New: Invalid optimization in the presence of shift instructions?

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Aug 9 03:58:19 PDT 2017


            Bug ID: 34133
           Summary: Invalid optimization in the presence of shift
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: c.cadar at imperial.ac.uk
                CC: llvm-bugs at lists.llvm.org

Consider the program below:

$ cat shift.c
int a = 0;
int foo(int x) { return (a < 2) || (a >> x) ; }

int main() {
  return foo(100);

$ clang -O1 -emit-llvm -c shift.c
$ llvm-dis shift.bc
$ cat shift.ll
define i32 @foo(i32) local_unnamed_addr #0 {
  %2 = load i32, i32* @a, align 4, !tbaa !1
  %3 = icmp slt i32 %2, 2
  %4 = ashr i32 %2, %0
  %5 = icmp ne i32 %4, 0
  %6 = or i1 %3, %5
  %7 = zext i1 %6 to i32
  ret i32 %7
define i32 @main() local_unnamed_addr #0 {
  %1 = tail call i32 @foo(i32 100)
  ret i32 %1

Note that in the C code, the || operator has short-circuiting
behaviour, so "a >> x" should not be evaluated since the first clause
is true.  However, the optimization generates code that always
executes the corresponding ashr instruction, which has undefined
behaviour in this case, as the shift amount is 100.  So is this
optimization valid?

We discovered this while working with KLEE, which generates an
overshift error when the code is compiled with -O1, but not with -O0,
so we'd like to understand whether KLEE's behaviour is correct here.
Note that UBSan does not complain here, as its instrumentation
disables the optimization.

Thank you,

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