optimize away fabs calls when input is known positive

[spatel]

Summary

This patch eliminates library calls and intrinsic calls to fabs when the input is a squared value.
This pattern will occur when a subsequent patch that I am working on to optimize sqrt calls is introduced, so I thought it'd be better to send this one out for review first because it's smaller.
Note that no unsafe-math / fast-math assumptions are needed for this optimization.

[beanz]

LGTM.

[spatel]

Thanks, Chris!

[spatel]

Closed by commit rL219717 (authored by @spatel).

llvm/trunk/include/llvm/Transforms/Utils/SimplifyLibCalls.h

private:
   Value *optimizeMemSet(CallInst *CI, IRBuilder<> &B);
 
   // Math Library Optimizations
   Value *optimizeUnaryDoubleFP(CallInst *CI, IRBuilder<> &B, bool CheckRetType);
   Value *optimizeBinaryDoubleFP(CallInst *CI, IRBuilder<> &B);
   Value *optimizeCos(CallInst *CI, IRBuilder<> &B);
   Value *optimizePow(CallInst *CI, IRBuilder<> &B);
   Value *optimizeExp2(CallInst *CI, IRBuilder<> &B);
+  Value *optimizeFabs(CallInst *CI, IRBuilder<> &B);
   Value *optimizeSinCosPi(CallInst *CI, IRBuilder<> &B);
 
   // Integer Library Call Optimizations
   Value *optimizeFFS(CallInst *CI, IRBuilder<> &B);
   Value *optimizeAbs(CallInst *CI, IRBuilder<> &B);
   Value *optimizeIsDigit(CallInst *CI, IRBuilder<> &B);
   Value *optimizeIsAscii(CallInst *CI, IRBuilder<> &B);
   Value *optimizeToAscii(CallInst *CI, IRBuilder<> &B);

llvm/trunk/lib/Transforms/Utils/SimplifyLibCalls.cpp

if (LdExpArg) {
         CI->setCallingConv(F->getCallingConv());
 
       return CI;
     }
   }
   return Ret;
 }
 
+Value *LibCallSimplifier::optimizeFabs(CallInst *CI, IRBuilder<> &B) {
+  Function *Callee = CI->getCalledFunction();
+
+  Value *Ret = nullptr;
+  if (Callee->getName() == "fabs" && TLI->has(LibFunc::fabsf)) {
+    Ret = optimizeUnaryDoubleFP(CI, B, false);
+  }
+
+  FunctionType *FT = Callee->getFunctionType();
+  // Make sure this has 1 argument of FP type which matches the result type.
+  if (FT->getNumParams() != 1 || FT->getReturnType() != FT->getParamType(0) ||
+      !FT->getParamType(0)->isFloatingPointTy())
+    return Ret;
+
+  Value *Op = CI->getArgOperand(0);
+  if (Instruction *I = dyn_cast<Instruction>(Op)) {
+    // Fold fabs(x * x) -> x * x; any squared FP value must already be positive.
+    if (I->getOpcode() == Instruction::FMul)
+      if (I->getOperand(0) == I->getOperand(1))
+        return Op;
+  }
+  return Ret;
+}
+
 static bool isTrigLibCall(CallInst *CI);
 static void insertSinCosCall(IRBuilder<> &B, Function *OrigCallee, Value *Arg,
                              bool UseFloat, Value *&Sin, Value *&Cos,
                              Value *&SinCos);
 
 Value *LibCallSimplifier::optimizeSinCosPi(CallInst *CI, IRBuilder<> &B) {
 
   // Make sure the prototype is as expected, otherwise the rest of the
Value *LibCallSimplifier::optimizeCall(CallInst *CI) {
   if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) {
     if (!isCallingConvC)
       return nullptr;
     switch (II->getIntrinsicID()) {
     case Intrinsic::pow:
       return optimizePow(CI, Builder);
     case Intrinsic::exp2:
       return optimizeExp2(CI, Builder);
+    case Intrinsic::fabs:
+      return optimizeFabs(CI, Builder);
     default:
       return nullptr;
     }
   }
 
   // Then check for known library functions.
   if (TLI->getLibFunc(FuncName, Func) && TLI->has(Func)) {
     // We never change the calling convention.
if (TLI->getLibFunc(FuncName, Func) && TLI->has(Func)) {
     case LibFunc::powf:
     case LibFunc::pow:
     case LibFunc::powl:
       return optimizePow(CI, Builder);
     case LibFunc::exp2l:
     case LibFunc::exp2:
     case LibFunc::exp2f:
       return optimizeExp2(CI, Builder);
+    case LibFunc::fabsf:
+    case LibFunc::fabs:
+    case LibFunc::fabsl:
+      return optimizeFabs(CI, Builder);
     case LibFunc::ffs:
     case LibFunc::ffsl:
     case LibFunc::ffsll:
       return optimizeFFS(CI, Builder);
     case LibFunc::abs:
     case LibFunc::labs:
     case LibFunc::llabs:
       return optimizeAbs(CI, Builder);
if (TLI->getLibFunc(FuncName, Func) && TLI->has(Func)) {
     case LibFunc::perror:
       return optimizeErrorReporting(CI, Builder);
     case LibFunc::vfprintf:
     case LibFunc::fiprintf:
       return optimizeErrorReporting(CI, Builder, 0);
     case LibFunc::fputc:
       return optimizeErrorReporting(CI, Builder, 1);
     case LibFunc::ceil:
-    case LibFunc::fabs:
     case LibFunc::floor:
     case LibFunc::rint:
     case LibFunc::round:
     case LibFunc::nearbyint:
     case LibFunc::trunc:
       if (hasFloatVersion(FuncName))
         return optimizeUnaryDoubleFP(CI, Builder, false);
       return nullptr;

llvm/trunk/test/Transforms/InstCombine/fabs.ll

+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; Make sure all library calls are eliminated when the input is known positive.
+
+declare float @fabsf(float)
+declare double @fabs(double)
+declare fp128 @fabsl(fp128)
+
+define float @square_fabs_call_f32(float %x) {
+  %mul = fmul float %x, %x
+  %fabsf = tail call float @fabsf(float %mul)
+  ret float %fabsf
+
+; CHECK-LABEL: square_fabs_call_f32(
+; CHECK-NEXT: %mul = fmul float %x, %x
+; CHECK-NEXT: ret float %mul
+}
+
+define double @square_fabs_call_f64(double %x) {
+  %mul = fmul double %x, %x
+  %fabs = tail call double @fabs(double %mul)
+  ret double %fabs
+
+; CHECK-LABEL: square_fabs_call_f64(
+; CHECK-NEXT: %mul = fmul double %x, %x
+; CHECK-NEXT: ret double %mul
+}
+
+define fp128 @square_fabs_call_f128(fp128 %x) {
+  %mul = fmul fp128 %x, %x
+  %fabsl = tail call fp128 @fabsl(fp128 %mul)
+  ret fp128 %fabsl
+
+; CHECK-LABEL: square_fabs_call_f128(
+; CHECK-NEXT: %mul = fmul fp128 %x, %x
+; CHECK-NEXT: ret fp128 %mul
+}
+
+; Make sure all intrinsic calls are eliminated when the input is known positive.
+
+declare float @llvm.fabs.f32(float)
+declare double @llvm.fabs.f64(double)
+declare fp128 @llvm.fabs.f128(fp128)
+
+define float @square_fabs_intrinsic_f32(float %x) {
+  %mul = fmul float %x, %x
+  %fabsf = tail call float @llvm.fabs.f32(float %mul)
+  ret float %fabsf
+
+; CHECK-LABEL: square_fabs_intrinsic_f32(
+; CHECK-NEXT: %mul = fmul float %x, %x
+; CHECK-NEXT: ret float %mul
+}
+
+define double @square_fabs_intrinsic_f64(double %x) {
+  %mul = fmul double %x, %x
+  %fabs = tail call double @llvm.fabs.f64(double %mul)
+  ret double %fabs
+
+; CHECK-LABEL: square_fabs_intrinsic_f64(
+; CHECK-NEXT: %mul = fmul double %x, %x
+; CHECK-NEXT: ret double %mul
+}
+
+define fp128 @square_fabs_intrinsic_f128(fp128 %x) {
+  %mul = fmul fp128 %x, %x
+  %fabsl = tail call fp128 @llvm.fabs.f128(fp128 %mul)
+  ret fp128 %fabsl
+
+; CHECK-LABEL: square_fabs_intrinsic_f128(
+; CHECK-NEXT: %mul = fmul fp128 %x, %x
+; CHECK-NEXT: ret fp128 %mul
+}
+
+; Shrinking a library call to a smaller type should not be inhibited by nor inhibit the square optimization.
+
+define float @square_fabs_shrink_call1(float %x) {
+  %ext = fpext float %x to double
+  %sq = fmul double %ext, %ext
+  %fabs = call double @fabs(double %sq)
+  %trunc = fptrunc double %fabs to float
+  ret float %trunc
+
+; CHECK-LABEL: square_fabs_shrink_call1(
+; CHECK-NEXT: %trunc = fmul float %x, %x
+; CHECK-NEXT: ret float %trunc
+}
+
+define float @square_fabs_shrink_call2(float %x) {
+  %sq = fmul float %x, %x
+  %ext = fpext float %sq to double
+  %fabs = call double @fabs(double %ext)
+  %trunc = fptrunc double %fabs to float
+  ret float %trunc
+
+; CHECK-LABEL: square_fabs_shrink_call2(
+; CHECK-NEXT: %sq = fmul float %x, %x
+; CHECK-NEXT: ret float %sq
+}
+