[ValueTracking] Improve impliesPoison

This patch improves ValueTracking's impliesPoison(V1, V2) to do this reasoning:

```
  %res = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %a, i64 %b)
  %overflow = extractvalue { i64, i1 } %res, 1
  %mul      = extractvalue { i64, i1 } %res, 0

	; If %mul is poison, %overflow is also poison, and vice versa.
```

This improvement leads to supporting this optimization under `-instcombine-unsafe-select-transform=0`:

```
define i1 @test2_logical(i64 %a, i64 %b, i64* %ptr) {
; CHECK-LABEL: @test2_logical(
; CHECK-NEXT:    [[MUL:%.*]] = mul i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT:    [[TMP1:%.*]] = icmp ne i64 [[A]], 0
; CHECK-NEXT:    [[TMP2:%.*]] = icmp ne i64 [[B]], 0
; CHECK-NEXT:    [[OVERFLOW_1:%.*]] = and i1 [[TMP1]], [[TMP2]]
; CHECK-NEXT:    [[NEG:%.*]] = sub i64 0, [[MUL]]
; CHECK-NEXT:    store i64 [[NEG]], i64* [[PTR:%.*]], align 8
; CHECK-NEXT:    ret i1 [[OVERFLOW_1]]
;

  %res = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %a, i64 %b)
  %overflow = extractvalue { i64, i1 } %res, 1
  %mul = extractvalue { i64, i1 } %res, 0
  %cmp = icmp ne i64 %mul, 0
  %overflow.1 = select i1 %overflow, i1 true, i1 %cmp
  %neg = sub i64 0, %mul
  store i64 %neg, i64* %ptr, align 8
  ret i1 %overflow.1
}
```

Previously, this didn't happen because the flag prevented `select i1 %overflow, i1 true, i1 %cmp` from being `or i1 %overflow, %cmp`.
Note that the select -> or conversion happens only when `impliesPoison(%cmp, %overflow)` returns true.
This improvement allows `impliesPoison` to do the reasoning.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D96929

0 files changed, 0 insertions, 0 deletions