Teach InstCombine to hoist FABS and FNEG through FPTRUNC instructions. The application of these operations commutes with the truncation, so we should prefer to do them in the smallest size we can, to save register space, use smaller constant pool entries, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172117 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 46 insertions, 1 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 5af444235c..c782032c45 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1204,8 +1204,34 @@ Instruction *InstCombiner::visitFPTrunc(FPTruncInst &CI) {
       }
       break;  
     }
+
+    // (fptrunc (fneg x)) -> (fneg (fptrunc x))
+    if (BinaryOperator::isFNeg(OpI)) {
+      Value *InnerTrunc = Builder->CreateFPTrunc(OpI->getOperand(1),
+                                                 CI.getType());
+      return BinaryOperator::CreateFNeg(InnerTrunc);
+    }
   }
-  
+
+  IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI.getOperand(0));
+  if (II) {
+    switch (II->getIntrinsicID()) {
+      default: break;
+      case Intrinsic::fabs: {
+        // (fptrunc (fabs x)) -> (fabs (fptrunc x))
+        Value *InnerTrunc = Builder->CreateFPTrunc(II->getArgOperand(0),
+                                                   CI.getType());
+        Type *IntrinsicType[] = { CI.getType() };
+        Function *Overload =
+          Intrinsic::getDeclaration(CI.getParent()->getParent()->getParent(),
+                                    II->getIntrinsicID(), IntrinsicType);
+
+        Value *Args[] = { InnerTrunc };
+        return CallInst::Create(Overload, Args, II->getName());
+      }
+    }
+  }
+
   // Fold (fptrunc (sqrt (fpext x))) -> (sqrtf x)
   CallInst *Call = dyn_cast<CallInst>(CI.getOperand(0));
   if (Call && Call->getCalledFunction() && TLI->has(LibFunc::sqrtf) &&
diff --git a/test/Transforms/InstCombine/fpcast.ll b/test/Transforms/InstCombine/fpcast.ll
index bc6aa0a689..09f053289d 100644
--- a/test/Transforms/InstCombine/fpcast.ll
+++ b/test/Transforms/InstCombine/fpcast.ll
@@ -13,3 +13,22 @@ define i8 @test2() {
 ; CHECK: ret i8 -1
 }
 
+; CHECK: test3
+define half @test3(float %a) {
+; CHECK: fptrunc
+; CHECK: llvm.fabs.f16
+  %b = call float @llvm.fabs.f32(float %a)
+  %c = fptrunc float %b to half
+  ret half %c
+}
+
+; CHECK: test4
+define half @test4(float %a) {
+; CHECK: fptrunc
+; CHECK: fsub
+  %b = fsub float -0.0, %a
+  %c = fptrunc float %b to half
+  ret half %c
+}
+
+declare float @llvm.fabs.f32(float) nounwind readonly