[InstCombine] fold fdiv with exp/exp2 divisor (PR49147)

Follow-up to:
D96648 / b40fde0
...for the special-case base calls.

From the earlier commit:
This is unusual in the general (non-reciprocal) case because we need
an extra instruction, but that should be better for general FP
reassociation and codegen. We conservatively check for "arcp" FMF
here as we do with existing fdiv folds, but it is not strictly
necessary to have that.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp

@@ -1326,8 +1326,9 @@ Instruction *InstCombinerImpl::visitFDiv(BinaryOperator &I) {
     if (match(Op1, m_FDiv(m_SpecificFP(1.0), m_Value(Y))))
       return BinaryOperator::CreateFMulFMF(Y, Op0, &I);
 
-    // Negate the exponent of pow to fold division-by-pow() into multiply:
+    // Negate the exponent of pow/exp to fold division-by-pow() into multiply:
     // Z / pow(X, Y) --> Z * pow(X, -Y)
+    // Z / exp{2}(Y) --> Z * exp{2}(-Y)
     // In the general case, this creates an extra instruction, but fmul allows
     // for better canonicalization and optimization than fdiv.
     if (match(Op1,
@@ -1336,6 +1337,16 @@ Instruction *InstCombinerImpl::visitFDiv(BinaryOperator &I) {
       Value *Pow = Builder.CreateBinaryIntrinsic(Intrinsic::pow, X, NegY, &I);
       return BinaryOperator::CreateFMulFMF(Op0, Pow, &I);
     }
+    if (match(Op1, m_OneUse(m_Intrinsic<Intrinsic::exp>(m_Value(Y))))) {
+      Value *NegY = Builder.CreateFNegFMF(Y, &I);
+      Value *Pow = Builder.CreateUnaryIntrinsic(Intrinsic::exp, NegY, &I);
+      return BinaryOperator::CreateFMulFMF(Op0, Pow, &I);
+    }
+    if (match(Op1, m_OneUse(m_Intrinsic<Intrinsic::exp2>(m_Value(Y))))) {
+      Value *NegY = Builder.CreateFNegFMF(Y, &I);
+      Value *Pow = Builder.CreateUnaryIntrinsic(Intrinsic::exp2, NegY, &I);
+      return BinaryOperator::CreateFMulFMF(Op0, Pow, &I);
+    }
   }
 
   if (I.hasAllowReassoc() && Op0->hasOneUse() && Op1->hasOneUse()) {

llvm/test/Transforms/InstCombine/fdiv.ll

@@ -731,8 +731,9 @@ define <2 x half> @pow_recip(<2 x half> %x, <2 x half> %y) {
 
 define float @exp_divisor(float %y, float %z) {
 ; CHECK-LABEL: @exp_divisor(
-; CHECK-NEXT:    [[P:%.*]] = call float @llvm.exp.f32(float [[Y:%.*]])
-; CHECK-NEXT:    [[R:%.*]] = fdiv reassoc arcp float [[Z:%.*]], [[P]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fneg reassoc arcp float [[Y:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = call reassoc arcp float @llvm.exp.f32(float [[TMP1]])
+; CHECK-NEXT:    [[R:%.*]] = fmul reassoc arcp float [[TMP2]], [[Z:%.*]]
 ; CHECK-NEXT:    ret float [[R]]
 ;
   %p = call float @llvm.exp.f32(float %y)
@@ -785,9 +786,9 @@ define float @exp_divisor_not_enough_fmf2(float %y, float %z) {
 
 define <2 x half> @exp_recip(<2 x half> %x, <2 x half> %y) {
 ; CHECK-LABEL: @exp_recip(
-; CHECK-NEXT:    [[P:%.*]] = call <2 x half> @llvm.exp.v2f16(<2 x half> [[Y:%.*]])
-; CHECK-NEXT:    [[R:%.*]] = fdiv reassoc ninf arcp <2 x half> <half 0xH3C00, half 0xH3C00>, [[P]]
-; CHECK-NEXT:    ret <2 x half> [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fneg reassoc ninf arcp <2 x half> [[Y:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = call reassoc ninf arcp <2 x half> @llvm.exp.v2f16(<2 x half> [[TMP1]])
+; CHECK-NEXT:    ret <2 x half> [[TMP2]]
 ;
   %p = call <2 x half> @llvm.exp.v2f16(<2 x half> %y)
   %r = fdiv reassoc arcp ninf <2 x half> <half 1.0, half 1.0>, %p
@@ -796,8 +797,9 @@ define <2 x half> @exp_recip(<2 x half> %x, <2 x half> %y) {
 
 define float @exp2_divisor(float %y, float %z) {
 ; CHECK-LABEL: @exp2_divisor(
-; CHECK-NEXT:    [[P:%.*]] = call float @llvm.exp2.f32(float [[Y:%.*]])
-; CHECK-NEXT:    [[R:%.*]] = fdiv reassoc arcp float [[Z:%.*]], [[P]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fneg reassoc arcp float [[Y:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = call reassoc arcp float @llvm.exp2.f32(float [[TMP1]])
+; CHECK-NEXT:    [[R:%.*]] = fmul reassoc arcp float [[TMP2]], [[Z:%.*]]
 ; CHECK-NEXT:    ret float [[R]]
 ;
   %p = call float @llvm.exp2.f32(float %y)
@@ -850,9 +852,9 @@ define float @exp2_divisor_not_enough_fmf2(float %y, float %z) {
 
 define <2 x half> @exp2_recip(<2 x half> %x, <2 x half> %y) {
 ; CHECK-LABEL: @exp2_recip(
-; CHECK-NEXT:    [[P:%.*]] = call <2 x half> @llvm.exp2.v2f16(<2 x half> [[Y:%.*]])
-; CHECK-NEXT:    [[R:%.*]] = fdiv reassoc ninf arcp <2 x half> <half 0xH3C00, half 0xH3C00>, [[P]]
-; CHECK-NEXT:    ret <2 x half> [[R]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fneg reassoc ninf arcp <2 x half> [[Y:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = call reassoc ninf arcp <2 x half> @llvm.exp2.v2f16(<2 x half> [[TMP1]])
+; CHECK-NEXT:    ret <2 x half> [[TMP2]]
 ;
   %p = call <2 x half> @llvm.exp2.v2f16(<2 x half> %y)
   %r = fdiv reassoc arcp ninf <2 x half> <half 1.0, half 1.0>, %p