IR: Stop requiring nsz to reassociate fmul

nsz can only change the behavior of the sign bit.
The sign bit for fmul can be implemented as xor,
which is associative. DAGCombiner already reassociates
the multiply by 2 constants without nsz.

Fixes #64967

Author: arsenm

llvm/lib/IR/Instruction.cpp

@@ -1271,6 +1271,7 @@ bool Instruction::isAssociative() const {
 
   switch (Opcode) {
   case FMul:
+    return cast<FPMathOperator>(this)->hasAllowReassoc();
   case FAdd:
     return cast<FPMathOperator>(this)->hasAllowReassoc() &&
            cast<FPMathOperator>(this)->hasNoSignedZeros();

llvm/test/Transforms/InstCombine/2006-10-26-VectorReassoc.ll

@@ -35,12 +35,10 @@ define <4 x float> @test_fmul_reassoc_nsz(<4 x float> %V) {
 }
 
 ; (V * C1) * C2 => V * (C1 * C2)
-; TODO: This doesn't require 'nsz'.  It should fold to V * { 1.0, 4.0e+05, -9.0, 16.0 }
 define <4 x float> @test_fmul_reassoc(<4 x float> %V) {
 ; CHECK-LABEL: @test_fmul_reassoc(
-; CHECK-NEXT:     [[TMP1:%.*]] = fmul reassoc <4 x float> [[V:%.*]], <float 1.000000e+00, float 2.000000e+00, float 3.000000e+00, float 4.000000e+00>
-; CHECK-NEXT:     [[TMP2:%.*]] = fmul reassoc <4 x float> [[TMP1]], <float 1.000000e+00, float 2.000000e+05, float -3.000000e+00, float 4.000000e+00>
-; CHECK-NEXT:     ret <4 x float> [[TMP2]]
+; CHECK: [[TMP1:%.*]] = fmul reassoc <4 x float> %V, <float 1.000000e+00, float 4.000000e+05, float -9.000000e+00, float 1.600000e+01>
+; CHECK-NEXT:     ret <4 x float> [[TMP1]]
         %Y = fmul reassoc <4 x float> %V, < float 1.000000e+00, float 2.000000e+00, float 3.000000e+00, float 4.000000e+00 >
         %Z = fmul reassoc <4 x float> %Y, < float 1.000000e+00, float 2.000000e+05, float -3.000000e+00, float 4.000000e+00 >
         ret <4 x float> %Z

llvm/test/Transforms/InstCombine/fdiv.ll

@@ -525,8 +525,7 @@ define <2 x float> @div_constant_dividend2_reassoc_only(<2 x float> %x) {
 
 define <2 x float> @div_constant_dividend3(<2 x float> %x) {
 ; CHECK-LABEL: @div_constant_dividend3(
-; CHECK-NEXT:    [[TMP1:%.*]] = fmul reassoc arcp <2 x float> [[X:%.*]], <float 1.500000e+01, float -7.000000e+00>
-; CHECK-NEXT:    [[T2:%.*]] = fmul reassoc arcp <2 x float> [[TMP1]], <float 0x3FD5555560000000, float 0x3FC24924A0000000>
+; CHECK-NEXT:    [[T2:%.*]] = fmul reassoc arcp <2 x float> [[X:%.*]], <float 5.000000e+00, float -1.000000e+00>
 ; CHECK-NEXT:    ret <2 x float> [[T2]]
 ;
   %t1 = fdiv <2 x float> <float 3.0e0, float 7.0e0>, %x

llvm/test/Transforms/InstCombine/issue64967-reassoc-fmul.ll

@@ -25,8 +25,7 @@ define float @fmul(float %x) {
 define float @fmul_reassoc(float %x) {
 ; CHECK-LABEL: define float @fmul_reassoc(
 ; CHECK-SAME: float [[X:%.*]]) {
-; CHECK-NEXT:    [[FMUL0:%.*]] = fmul reassoc float [[X]], 2.000000e+00
-; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[FMUL0]], 4.000000e+00
+; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[X]], 8.000000e+00
 ; CHECK-NEXT:    ret float [[FMUL1]]
 ;
   %fmul0 = fmul reassoc float %x, 2.0
@@ -37,8 +36,7 @@ define float @fmul_reassoc(float %x) {
 define <2 x float> @fmul_reassoc_v2(<2 x float> %x) {
 ; CHECK-LABEL: define <2 x float> @fmul_reassoc_v2(
 ; CHECK-SAME: <2 x float> [[X:%.*]]) {
-; CHECK-NEXT:    [[FMUL0:%.*]] = fmul reassoc <2 x float> [[X]], splat (float 2.000000e+00)
-; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc <2 x float> [[FMUL0]], splat (float 4.000000e+00)
+; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc <2 x float> [[X]], splat (float 8.000000e+00)
 ; CHECK-NEXT:    ret <2 x float> [[FMUL1]]
 ;
   %fmul0 = fmul reassoc <2 x float> %x, splat (float 2.0)
@@ -54,8 +52,7 @@ define <2 x float> @fmul_reassoc_v2(<2 x float> %x) {
 define float @fmul_reassoc_negative_0(float %x) {
 ; CHECK-LABEL: define float @fmul_reassoc_negative_0(
 ; CHECK-SAME: float [[X:%.*]]) {
-; CHECK-NEXT:    [[FMUL0:%.*]] = fmul reassoc float [[X]], 2.000000e+00
-; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[FMUL0]], -4.000000e+00
+; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[X]], -8.000000e+00
 ; CHECK-NEXT:    ret float [[FMUL1]]
 ;
   %fmul0 = fmul reassoc float %x, 2.0
@@ -71,8 +68,7 @@ define float @fmul_reassoc_negative_0(float %x) {
 define float @fmul_reassoc_negative_1(float %x) {
 ; CHECK-LABEL: define float @fmul_reassoc_negative_1(
 ; CHECK-SAME: float [[X:%.*]]) {
-; CHECK-NEXT:    [[FMUL0:%.*]] = fmul reassoc float [[X]], -2.000000e+00
-; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[FMUL0]], 4.000000e+00
+; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[X]], -8.000000e+00
 ; CHECK-NEXT:    ret float [[FMUL1]]
 ;
   %fmul0 = fmul reassoc float %x, -2.0
@@ -95,8 +91,7 @@ define float @fmul_reassoc_nsz(float %x) {
 define float @fmul_reassoc_posk_neg0(float %x) {
 ; CHECK-LABEL: define float @fmul_reassoc_posk_neg0(
 ; CHECK-SAME: float [[X:%.*]]) {
-; CHECK-NEXT:    [[FMUL0:%.*]] = fmul reassoc float [[X]], 4.000000e+00
-; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[FMUL0]], -0.000000e+00
+; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[X]], -0.000000e+00
 ; CHECK-NEXT:    ret float [[FMUL1]]
 ;
   %fmul0 = fmul reassoc float %x, 4.0
@@ -108,8 +103,7 @@ define float @fmul_reassoc_neg0_posk(float %x) {
 ; CHECK-LABEL: define float @fmul_reassoc_neg0_posk(
 ; CHECK-SAME: float [[X:%.*]]) {
 ; CHECK-NEXT:    [[FMUL0:%.*]] = fmul reassoc float [[X]], -0.000000e+00
-; CHECK-NEXT:    [[FMUL1:%.*]] = fmul reassoc float [[FMUL0]], 4.000000e+00
-; CHECK-NEXT:    ret float [[FMUL1]]
+; CHECK-NEXT:    ret float [[FMUL0]]
 ;
   %fmul0 = fmul reassoc float %x, -0.0
   %fmul1 = fmul reassoc float %fmul0, 4.0