[llvm-bugs] [Bug 44100] New: Missed canonicalizations for losslessness checks of unsigned integer demotions

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44100

            Bug ID: 44100
           Summary: Missed canonicalizations for losslessness checks of
                    unsigned integer demotions
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

Given

unsigned short t0(unsigned short x) {
  x+=1;
  return x;
}
unsigned short t1(unsigned short x) {
  x-=1;
  return x;
}

With -fsanitize=implicit-conversion we produce:

  %3 = add nuw nsw i32 %2, 1, !dbg !15
  %4 = trunc i32 %3 to i16, !dbg !15
  %5 = icmp ult i32 %3, 65536, !dbg !15

and

  %2 = zext i16 %0 to i32, !dbg !22
  %3 = add nsw i32 %2, -1, !dbg !22
  %4 = trunc i32 %3 to i16, !dbg !22
  %5 = icmp ult i32 %3, 65536, !dbg !22

https://godbolt.org/z/Us33sA

But in these cases we can simply check the original %2:

Name: unsigned truncation check - increment
Pre: C2 u>= C1
  %add = add nuw i32 %conv, C1 ; has extra uses
  %cmp = icmp ult i32 %add, C2 ; is truncation NUW?
=>
  %cmp = icmp ult i32 %conv, (C2-C1)

Name: unsigned truncation check - decrement
Pre: C1 <= 0 ; C1 is non-positive
  %conv = zext i16 %x to i32
  %add = add i32 %conv, C1 ; has extra uses
  %cmp = icmp ult i32 %add, 65536 ; is truncation NUW?
=>
  %cmp = icmp uge i32 %conv, -C1

https://rise4fun.com/Alive/x35G

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