[PATCH] D28479: [SimplifyLibCalls] pow(x, -0.5) -> 1.0 / sqrt(x)

[Davide Italiano via Phabricator via llvm-commits]

davide created this revision.
davide added reviewers: spatel, majnemer.
davide added a subscriber: llvm-commits.

Missing optimization pointed out by David earlier https://godbolt.org/g/PsWrfV


https://reviews.llvm.org/D28479

Files:
  lib/Transforms/Utils/SimplifyLibCalls.cpp
  test/Transforms/InstCombine/pow-sqrt.ll


Index: test/Transforms/InstCombine/pow-sqrt.ll
===================================================================
--- test/Transforms/InstCombine/pow-sqrt.ll
+++ test/Transforms/InstCombine/pow-sqrt.ll
@@ -9,5 +9,14 @@
 ; CHECK-NEXT:  %sqrt = call fast double @sqrt(double %x)
 ; CHECK-NEXT:  ret double %sqrt
 
-declare double @llvm.pow.f64(double, double)
+define double @pow_neghalf(double %x) {
+  %pow = call fast double @llvm.pow.f64(double %x, double -5.000000e-01)
+  ret double %pow
+}
 
+; CHECK-LABEL: define double @pow_neghalf(
+; CHECK-NEXT: %sqrt = call double @sqrt(double %x) #0
+; CHECK-NEXT: %sqrtrecip = fdiv double 1.000000e+00, %sqrt
+; CHECK-NEXT: ret double %sqrtrecip
+
+declare double @llvm.pow.f64(double, double)
Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1074,6 +1074,22 @@
   if (Op2C->getValueAPF().isZero()) // pow(x, 0.0) -> 1.0
     return ConstantFP::get(CI->getType(), 1.0);
 
+  if (Op2C->isExactlyValue(-0.5) &&
+      hasUnaryFloatFn(TLI, Op2->getType(), LibFunc::sqrt, LibFunc::sqrtf,
+                      LibFunc::sqrtl)) {
+    // If -ffast-math:
+    // pow(x, -0.5) -> 1.0 / sqrt(x)
+    if (CI->hasUnsafeAlgebra()) {
+
+      // Here we cannot lower to an intrinsic because C99 sqrt() and llvm.sqrt
+      // are not guaranteed to have the same semantics.
+      Value *Sqrt = emitUnaryFloatFnCall(Op1, TLI->getName(LibFunc::sqrt), B,
+                                         Callee->getAttributes());
+
+      return B.CreateFDiv(ConstantFP::get(CI->getType(), 1.0), Sqrt, "sqrtrecip");
+    }
+  }
+
   if (Op2C->isExactlyValue(0.5) &&
       hasUnaryFloatFn(TLI, Op2->getType(), LibFunc::sqrt, LibFunc::sqrtf,
                       LibFunc::sqrtl) &&


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