[LLVMbugs] [Bug 20997] New: undefined behavior introduced by optimizer

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20997

            Bug ID: 20997
           Summary: undefined behavior introduced by optimizer
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: regehr at cs.utah.edu
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The C program below does not execute any undefined behaviors. However, the LLVM
code executes an undefined operation in the "sub nsw" which poisons the rest of
the computation.


regehr at regehr-M51AC:souper-bug1$ cat small.c
int printf(const char *, ...);
int x0 = 4294967295, x1;
int main() {
  x1 = x0 > 0 && 1 > 2147483647 - x0 || 1 + x0;
  if (x1)
    printf("x");
  return 0;
}

regehr at regehr-M51AC:souper-bug1$ clang -O2 -w small.c -S -o - -emit-llvm
; ModuleID = 'small.c'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@x0 = global i32 -1, align 4
@x1 = common global i32 0, align 4

; Function Attrs: nounwind uwtable
define i32 @main() #0 {
entry:
  %0 = load i32* @x0, align 4, !tbaa !1
  %cmp = icmp sgt i32 %0, 0
  %sub = sub nsw i32 2147483647, %0
  %cmp1 = icmp slt i32 %sub, 1
  %or.cond = and i1 %cmp, %cmp1
  br i1 %or.cond, label %lor.end.thread, label %lor.end

lor.end.thread:                                   ; preds = %entry
  store i32 1, i32* @x1, align 4, !tbaa !1
  br label %if.then

lor.end:                                          ; preds = %entry
  %tobool = icmp ne i32 %0, -1
  %lor.ext = zext i1 %tobool to i32
  store i32 %lor.ext, i32* @x1, align 4, !tbaa !1
  br i1 %tobool, label %if.then, label %if.end

if.then:                                          ; preds = %lor.end.thread,
%lor.end
  %putchar = tail call i32 @putchar(i32 120) #1
  br label %if.end

if.end:                                           ; preds = %if.then, %lor.end
  ret i32 0
}

; Function Attrs: nounwind
declare i32 @putchar(i32) #1

attributes #0 = { nounwind uwtable "less-precise-fpmad"="false"
"no-frame-pointer-elim"="false" "no-infs-fp-math"="false"
"no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
"unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { nounwind }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"clang version 3.6.0 (217983)"}
!1 = metadata !{metadata !2, metadata !2, i64 0}
!2 = metadata !{metadata !"int", metadata !3, i64 0}
!3 = metadata !{metadata !"omnipotent char", metadata !4, i64 0}
!4 = metadata !{metadata !"Simple C/C++ TBAA"}

regehr at regehr-M51AC:souper-bug1$ clang -v
clang version 3.6.0 (217983)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.9.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.1
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
regehr at regehr-M51AC:souper-bug1$

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20140918/9e0ca436/attachment.html>