[llvm-bugs] [Bug 141822] Is CMN safe when the subtraction is nsw?

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Issue	141822
Summary	Is CMN safe when the subtraction is nsw?
Labels	new issue
Assignees
Reporter	AZero13

    In C, 

bool cmp(int a, int b) {
return a > -b;
}

yeilds

define i1 @cmn_nsw(i32 %a, i32 %b) {
; CHECK-LABEL: cmn_nsw:
; CHECK:       // %bb.0:
; CHECK-NEXT:    neg w8, w1
; CHECK-NEXT:    cmp w0, w8
; CHECK-NEXT:    cset w0, gt
; CHECK-NEXT: ret
  %sub = sub nsw i32 0, %b
  %cmp = icmp sgt i32 %a, %sub
  ret i1 %cmp
}

Now, because no fwrapv, the sub is assumed to be nsw.

However, this would mean no signed wrap.

For 0 - b to be signed wrap, b would have to be a value that fits in the realm of possible signed integers, and cause a wrap.

The only value for b that fits is 0x80000000, and therefore:
Iff b is 0x80000000, then 0 - b is signed wrap.

Therefore,
if 0 - b is not signed wrap, or nsw, b is not 0x80000000

Which means in theory I could do this:
define i1 @cmn_nsw(i32 %a, i32 %b) {
; CHECK-LABEL: cmn_nsw:
; CHECK:       // %bb.0:
; CHECK-NEXT:    cmn w0, w1
; CHECK-NEXT:    cset w0, gt
; CHECK-NEXT:    ret
  %sub = sub nsw i32 0, %b
  %cmp = icmp sgt i32 %a, %sub
  ret i1 %cmp
}

However, alive2 says:

define i1 @src(i32 %a, i32 %b) {
; CHECK-LABEL: cmn_nsw:
; CHECK:       // %bb.0:
; CHECK-NEXT:    cmn w0, w1
; CHECK-NEXT:    cset w0, gt
; CHECK-NEXT: ret
  %sub = sub nsw i32 0, %b
  %cmp = icmp sgt i32 %a, %sub
  ret i1 %cmp
}

define i1 @tgt(i32 %a, i32 %b) {
; CHECK-LABEL: cmn_nsw:
; CHECK:       // %bb.0:
; CHECK-NEXT:    cmn w0, w1
; CHECK-NEXT:    cset w0, gt
; CHECK-NEXT:    ret
  %isNotMin = icmp ne i32 %b, -2147483648
 call void @llvm.assume(i1 %isNotMin)
  %sub = sub i32 0, %b
  %cmp = icmp sgt i32 %a, %sub
  ret i1 %cmp
}

ERROR: Source is more defined than target

Example:
i32 %a = #x00000003 (3)
i32 %b = #x80000000 (2147483648, -2147483648)

Source:
i32 %sub = poison
i1 %cmp = poison

Target:
i1 %isNotMin = #x0 (0)
void = UB triggered!

I know I am thinking of multiple levels at once, but the question is, if I am checking if it is safe to turn cmp into cmn, is it safe to just look at the no signed wrap flag?

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