[InstSimplify] propagate poison through FP ops

We already have this fold:
  fadd float poison, 1.0 --> poison
...via ConstantFolding, so this makes the behavior consistent
if the other operand(s) are non-constant.

The fold for undef was added before poison existed as a
value/type in IR.

This came up in D102673 / D103169
because we're trying to sort out the more complicated handling
for constrained math ops.
We should have the handling for the regular instructions done
first, so we can build on that (or diverge as needed).

Differential Revision: https://reviews.llvm.org/D104383

lib/Analysis/InstructionSimplify.cpp

@@ -4807,12 +4807,16 @@ static Constant *propagateNaN(Constant *In) {
 }
 
 /// Perform folds that are common to any floating-point operation. This implies
-/// transforms based on undef/NaN because the operation itself makes no
+/// transforms based on poison/undef/NaN because the operation itself makes no
 /// difference to the result.
-static Constant *simplifyFPOp(ArrayRef<Value *> Ops,
-                              FastMathFlags FMF,
+static Constant *simplifyFPOp(ArrayRef<Value *> Ops, FastMathFlags FMF,
                               const SimplifyQuery &Q) {
   for (Value *V : Ops) {
+    // Poison is independent of anything else. It always propagates from an
+    // operand to a math result.
+    if (match(V, m_Poison()))
+      return PoisonValue::get(V->getType());
+
     bool IsNan = match(V, m_NaN());
     bool IsInf = match(V, m_Inf());
     bool IsUndef = Q.isUndefValue(V);

test/Transforms/InstSimplify/call.ll

@@ -953,7 +953,7 @@ define double @fma_undef_op0(double %x, double %y) {
 
 define double @fma_poison_op0(double %x, double %y) {
 ; CHECK-LABEL: @fma_poison_op0(
-; CHECK-NEXT:    ret double 0x7FF8000000000000
+; CHECK-NEXT:    ret double poison
 ;
   %r = call double @llvm.fma.f64(double poison, double %x, double %y)
   ret double %r
@@ -969,7 +969,7 @@ define double @fma_undef_op1(double %x, double %y) {
 
 define double @fma_poison_op1(double %x, double %y) {
 ; CHECK-LABEL: @fma_poison_op1(
-; CHECK-NEXT:    ret double 0x7FF8000000000000
+; CHECK-NEXT:    ret double poison
 ;
   %r = call double @llvm.fma.f64(double %x, double poison, double %y)
   ret double %r
@@ -985,7 +985,7 @@ define double @fma_undef_op2(double %x, double %y) {
 
 define double @fma_poison_op2(double %x, double %y) {
 ; CHECK-LABEL: @fma_poison_op2(
-; CHECK-NEXT:    ret double 0x7FF8000000000000
+; CHECK-NEXT:    ret double poison
 ;
   %r = call double @llvm.fma.f64(double %x, double %y, double poison)
   ret double %r
@@ -1001,7 +1001,7 @@ define double @fmuladd_undef_op0(double %x, double %y) {
 
 define double @fmuladd_poison_op0(double %x, double %y) {
 ; CHECK-LABEL: @fmuladd_poison_op0(
-; CHECK-NEXT:    ret double 0x7FF8000000000000
+; CHECK-NEXT:    ret double poison
 ;
   %r = call double @llvm.fmuladd.f64(double poison, double %x, double %y)
   ret double %r
@@ -1017,7 +1017,7 @@ define double @fmuladd_undef_op1(double %x, double %y) {
 
 define double @fmuladd_poison_op1(double %x, double %y) {
 ; CHECK-LABEL: @fmuladd_poison_op1(
-; CHECK-NEXT:    ret double 0x7FF8000000000000
+; CHECK-NEXT:    ret double poison
 ;
   %r = call double @llvm.fmuladd.f64(double %x, double poison, double %y)
   ret double %r
@@ -1033,7 +1033,7 @@ define double @fmuladd_undef_op2(double %x, double %y) {
 
 define double @fmuladd_poison_op2(double %x, double %y) {
 ; CHECK-LABEL: @fmuladd_poison_op2(
-; CHECK-NEXT:    ret double 0x7FF8000000000000
+; CHECK-NEXT:    ret double poison
 ;
   %r = call double @llvm.fmuladd.f64(double %x, double %y, double poison)
   ret double %r

test/Transforms/InstSimplify/fp-undef-poison.ll

@@ -1,8 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s -instsimplify -S | FileCheck %s
 
-; TODO: the instructions with poison operands should return poison
-
 define float @fadd_undef_op0(float %x) {
 ; CHECK-LABEL: @fadd_undef_op0(
 ; CHECK-NEXT:    ret float 0x7FF8000000000000
@@ -13,7 +11,7 @@ define float @fadd_undef_op0(float %x) {
 
 define float @fadd_poison_op0(float %x) {
 ; CHECK-LABEL: @fadd_poison_op0(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fadd float poison, %x
   ret float %r
@@ -29,7 +27,7 @@ define float @fadd_undef_op1(float %x) {
 
 define float @fadd_poison_op1(float %x) {
 ; CHECK-LABEL: @fadd_poison_op1(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fadd float %x, poison
   ret float %r
@@ -45,7 +43,7 @@ define float @fsub_undef_op0(float %x) {
 
 define float @fsub_poison_op0(float %x) {
 ; CHECK-LABEL: @fsub_poison_op0(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fsub float poison, %x
   ret float %r
@@ -61,7 +59,7 @@ define float @fsub_undef_op1(float %x) {
 
 define float @fsub_poison_op1(float %x) {
 ; CHECK-LABEL: @fsub_poison_op1(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fsub float %x, poison
   ret float %r
@@ -77,7 +75,7 @@ define float @fmul_undef_op0(float %x) {
 
 define float @fmul_poison_op0(float %x) {
 ; CHECK-LABEL: @fmul_poison_op0(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fmul float poison, %x
   ret float %r
@@ -93,7 +91,7 @@ define float @fmul_undef_op1(float %x) {
 
 define float @fmul_poison_op1(float %x) {
 ; CHECK-LABEL: @fmul_poison_op1(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fmul float %x, poison
   ret float %r
@@ -109,7 +107,7 @@ define float @fdiv_undef_op0(float %x) {
 
 define float @fdiv_poison_op0(float %x) {
 ; CHECK-LABEL: @fdiv_poison_op0(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fdiv float poison, %x
   ret float %r
@@ -125,7 +123,7 @@ define float @fdiv_undef_op1(float %x) {
 
 define float @fdiv_poison_op1(float %x) {
 ; CHECK-LABEL: @fdiv_poison_op1(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = fdiv float %x, poison
   ret float %r
@@ -141,7 +139,7 @@ define float @frem_undef_op0(float %x) {
 
 define float @frem_poison_op0(float %x) {
 ; CHECK-LABEL: @frem_poison_op0(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = frem float poison, %x
   ret float %r
@@ -157,7 +155,7 @@ define float @frem_undef_op1(float %x) {
 
 define float @frem_poison_op1(float %x) {
 ; CHECK-LABEL: @frem_poison_op1(
-; CHECK-NEXT:    ret float 0x7FF8000000000000
+; CHECK-NEXT:    ret float poison
 ;
   %r = frem float %x, poison
   ret float %r