[llvm] [InstCombine] Implement `fcmp (fadd x, 0.0), y` => `fcmp x, y` optimization (PR #88476)

[Vlad Mishel via llvm-commits]

================
@@ -1284,3 +1284,175 @@ define <1 x i1> @bitcast_1vec_eq0(i32 %x) {
   %cmp = fcmp oeq <1 x float> %f, zeroinitializer
   ret <1 x i1> %cmp
 }
+
+; Simplify fcmp (x + 0.0), y => fcmp x, y
+
+define i1 @fcmp_fadd_zero_ugt(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_ugt(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ugt float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp ugt float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_uge(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_uge(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp uge float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp uge float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_ogt(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_ogt(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ogt float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp ogt float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_oge(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_oge(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp oge float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp oge float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_ult(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_ult(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ult float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp ult float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_ule(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_ule(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ule float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp ule float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_olt(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_olt(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp olt float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp olt float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_ole(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_ole(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ole float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp ole float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_oeq(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_oeq(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp oeq float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp oeq float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_one(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_one(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp one float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp one float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_ueq(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_ueq(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ueq float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp ueq float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_une(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_une(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp une float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp une float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_ord(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_ord(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ord float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp ord float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_uno(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_uno(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp uno float [[ADD:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %x, 0.000000e+00
+  %cmp = fcmp uno float %add, %y
+  ret i1 %cmp
+}
+
+define i1 @fcmp_fadd_zero_switched(float %x, float %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_switched(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ult float [[Y:%.*]], [[X:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %add = fadd float %y, 0.000000e+00
+  %cmp = fcmp ugt float %x, %add
+  ret i1 %cmp
+}
+
+define <2 x i1> @fcmp_fadd_zero_vec(<2 x float> %x, <2 x float> %y) {
+; CHECK-LABEL: @fcmp_fadd_zero_vec(
+; CHECK-NEXT:    [[CMP:%.*]] = fcmp ugt <2 x float> [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+;
+  %add = fadd <2 x float> %x, <float 0.0, float -0.0>
+  %cmp = fcmp ugt <2 x float> %add, %y
+  ret <2 x i1> %cmp
+}
+
+define i1 @fcmp_fadd_zero_fast(float %x, float %y) {
----------------
vmishelcs wrote:

> Specifically you have to make sure the flags don't come from the fadd, they cannot be preserved

Added! The behaviour seems to be correct from what I can tell.

I have a quick question: How come for some of the tests, the `CHECK` lines generate as
```
define i1 @fcmp_fadd_zero_ugt(float %x, float %y) {
; CHECK-LABEL: @fcmp_fadd_zero_ugt(
; CHECK-NEXT:    [[CMP:%.*]] = fcmp ugt float [[ADD:%.*]], [[Y:%.*]]
; CHECK-NEXT:    ret i1 [[CMP]]
;
  %add = fadd float %x, 0.000000e+00
  %cmp = fcmp ugt float %add, %y
  ret i1 %cmp
}
```
and for the tests which I added later, the `CHECK` lines are
```
define i1 @fcmp_fadd_neg_zero(float %x, float %y) {
; CHECK-LABEL: @fcmp_fadd_neg_zero(
; CHECK-NEXT:    [[CMP:%.*]] = fcmp ugt float [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT:    ret i1 [[CMP]]
;
  %add = fadd float %x, -0.000000e+00
  %cmp = fcmp ugt float %add, %y
  ret i1 %cmp
}
```
?
In particular, why is it that for the former test, the `CHECK` line uses `[[ADD:%.*]]` but the latter uses `[[X:%.*]]`? When I ran the `opt` binary, the result is correct in both cases, but I was wondering why the slight difference.

https://github.com/llvm/llvm-project/pull/88476