[InstCombine] Fix issue in the simplification of pow() with nested exp{,2}()

[evandro]

Summary

Fix the issue of duplicating the call to exp{,2}() when it's nested in pow(), as exposed by rL340462.

[spatel]

It isn't obvious why this case is different than the other transforms, so some code comments are needed.

1. We can't rely on dead code elimination because libcalls may not be readnone (ie, they may set errno).
2. The calling code in LibCallSimplifier and InstCombine handles erasing the called function itself in all cases.
3. In this transform, we're potentially eliminating the intermediate exp2 call, so we have to manually kill it.

But what happens if the exp2 call has another use besides the pow call? We need a test for that scenario.

[evandro]

The function pow_exp2_libcall() in the test case is one such example without readnone. I'll augment the test case with a reuse scenario.

[evandro]

Address the case when the result from exp{,2}() is reused elsewhere.

[spatel]

LGTM

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

Value *LibCallSimplifier::replacePowWithExp(CallInst *Pow, IRBuilder<> &B) {
   AttributeList Attrs = Pow->getCalledFunction()->getAttributes();
   Module *Mod = Pow->getModule();
   Type *Ty = Pow->getType();
 
   // Evaluate special cases related to a nested function as the base.
 
   // pow(exp(x), y) -> exp(x * y)
   // pow(exp2(x), y) -> exp2(x * y)
-  // Only with fully relaxed math semantics, since, besides rounding
-  // differences, the transformation changes overflow and underflow behavior
-  // quite dramatically.
-  // For example:
+  // If exp{,2}() is used only once, it is better to fold two transcendental
+  // math functions into one.  If used again, exp{,2}() would still have to be
+  // called with the original argument, then keep both original transcendental
+  // functions.  However, this transformation is only safe with fully relaxed
+  // math semantics, since, besides rounding differences, it changes overflow
+  // and underflow behavior quite dramatically.  For example:
   //   pow(exp(1000), 0.001) = pow(inf, 0.001) = inf
   // Whereas:
   //   exp(1000 * 0.001) = exp(1)
   // TODO: Loosen the requirement for fully relaxed math semantics.
   // TODO: Handle exp10() when more targets have it available.
   CallInst *BaseFn = dyn_cast<CallInst>(Base);
-  if (BaseFn && BaseFn->isFast() && Pow->isFast()) {
+  if (BaseFn && BaseFn->hasOneUse() && BaseFn->isFast() && Pow->isFast()) {
     LibFunc LibFn;
+
     Function *CalleeFn = BaseFn->getCalledFunction();
     if (CalleeFn && TLI->getLibFunc(CalleeFn->getName(), LibFn) &&
         (LibFn == LibFunc_exp || LibFn == LibFunc_exp2) && TLI->has(LibFn)) {
+      Value *ExpFn;
+
+      // Create new exp{,2}() with the product as its argument.
       Value *FMul = B.CreateFMul(BaseFn->getArgOperand(0), Expo, "mul");
-      return emitUnaryFloatFnCall(FMul, CalleeFn->getName(), B, Attrs);
+      ExpFn = emitUnaryFloatFnCall(FMul, CalleeFn->getName(), B,
+                                   BaseFn->getAttributes());
+
+      // Since the new exp{,2}() is different from the original one, dead code
+      // elimination cannot be trusted to remove it, since it may have side
+      // effects (e.g., errno).  When the only consumer for the original
+      // exp{,2}() is pow(), then it has to be explicitly erased.
+      BaseFn->replaceAllUsesWith(ExpFn);
+      BaseFn->eraseFromParent();
+
+      return ExpFn;
     }
   }
 
   // Evaluate special cases related to a constant base.
 
   // pow(2.0, x) -> exp2(x)
   if (match(Base, m_SpecificFP(2.0))) {
     Value *Exp2 = Intrinsic::getDeclaration(Mod, Intrinsic::exp2, Ty);

llvm/test/Transforms/InstCombine/pow-exp.ll

 ; CHECK-NEXT:    [[EXP2:%.*]] = call fast double @exp2(double [[MUL]]) #1
 ; CHECK-NEXT:    ret double [[EXP2]]
 ;
   %call = call fast double @exp2(double %x) nounwind readnone
   %pow = call fast double @llvm.pow.f64(double %call, double %y)
   ret double %pow
 }
 
-; FIXME: Should not result in two calls to exp2().
 define double @pow_exp2_libcall(double %x, double %y) {
 ; CHECK-LABEL: @pow_exp2_libcall(
-; CHECK-NEXT:    [[CALL:%.*]] = call fast double @exp2(double [[X:%.*]])
 ; CHECK-NEXT:    [[MUL:%.*]] = fmul fast double [[X]], [[Y:%.*]]
 ; CHECK-NEXT:    [[EXP2:%.*]] = call fast double @exp2(double [[MUL]])
 ; CHECK-NEXT:    ret double [[EXP2]]
 ;
   %call = call fast double @exp2(double %x)
   %pow = call fast double @pow(double %call, double %y)
   ret double %pow
 }

 ; CHECK-NEXT:    [[POW:%.*]] = call fast fp128 @llvm.pow.f128(fp128 [[CALL]], fp128 [[Y:%.*]])
 ; CHECK-NEXT:    ret fp128 [[POW]]
 ;
   %call = call fast fp128 @exp10l(fp128 %x) nounwind readnone
   %pow = call fast fp128 @llvm.pow.f128(fp128 %call, fp128 %y)
   ret fp128 %pow
 }
 
+define float @reuse_fast(float %x, float %y, float * %p) {
+; CHECK-LABEL: @reuse_fast(
+; CHECK-NEXT:    [[EXP:%.*]] = call fast float @expf(float [[X:%.*]])
+; CHECK-NEXT:    [[POW:%.*]] = call fast float @powf(float [[EXP]], float [[Y:%.*]])
+; CHECK-NEXT:    store float [[EXP]], float* [[P:%.*]], align 4
+; CHECK-NEXT:    ret float [[POW]]
+;
+  %exp = call fast float @expf(float %x)
+  %pow = call fast float @powf(float %exp, float %y)
+  store float %exp, float *%p, align 4
+  ret float %pow
+}
+
+define fp128 @reuse_libcall(fp128 %x, fp128 %y, fp128 * %p) {
+; CHECK-LABEL: @reuse_libcall(
+; CHECK-NEXT:    [[EXP:%.*]] = call fp128 @expl(fp128 [[X:%.*]])
+; CHECK-NEXT:    [[POW:%.*]] = call fp128 @powl(fp128 [[EXP]], fp128 [[Y:%.*]])
+; CHECK-NEXT:    store fp128 [[EXP]], fp128* [[P:%.*]], align 16
+; CHECK-NEXT:    ret fp128 [[POW]]
+;
+  %exp = call fp128 @expl(fp128 %x)
+  %pow = call fp128 @powl(fp128 %exp, fp128 %y)
+  store fp128 %exp, fp128 *%p, align 16
+  ret fp128 %pow
+}
+
 define double @function_pointer(double ()* %fptr, double %p1) {
 ; CHECK-LABEL: @function_pointer(
 ; CHECK-NEXT:    [[CALL1:%.*]] = call fast double [[FPTR:%.*]]()
 ; CHECK-NEXT:    [[POW:%.*]] = call fast double @llvm.pow.f64(double [[CALL1]], double [[P1:%.*]])
 ; CHECK-NEXT:    ret double [[POW]]
 ;
   %call1 = call fast double %fptr()
   %pow = call fast double @llvm.pow.f64(double %call1, double %p1)