Pull fptrunc's upwards through selects when one of the select's selectands was a constant. This has a number of benefits, including producing small immediates (easier to materialize, smaller constant pools) as well as being more likely to allow the fptrunc to fuse with a preceding instruction (truncating selects are unusual).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191929 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -1229,6 +1229,19 @@ Instruction *InstCombiner::visitFPTrunc(FPTruncInst &CI) {
     }
   }
 
+  // (fptrunc (select cond, R1, Cst)) -->
+  // (select cond, (fptrunc R1), (fptrunc Cst))
+  SelectInst *SI = dyn_cast<SelectInst>(CI.getOperand(0));
+  if (SI &&
+      (isa<ConstantFP>(SI->getOperand(1)) ||
+       isa<ConstantFP>(SI->getOperand(2)))) {
+    Value *LHSTrunc = Builder->CreateFPTrunc(SI->getOperand(1),
+                                             CI.getType());
+    Value *RHSTrunc = Builder->CreateFPTrunc(SI->getOperand(2),
+                                             CI.getType());
+    return SelectInst::Create(SI->getOperand(0), LHSTrunc, RHSTrunc);
+  }
+
   IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI.getOperand(0));
   if (II) {
     switch (II->getIntrinsicID()) {

test/Transforms/InstCombine/fpcast.ll

@@ -31,4 +31,16 @@ define half @test4(float %a) {
   ret half %c
 }
 
+; CHECK: test5
+define half @test5(float %a, float %b, float %c) {
+; CHECK: fcmp ogt
+; CHECK: fptrunc
+; CHECK: select
+; CHECK: half 0xH3C00
+  %d = fcmp ogt float %a, %b
+  %e = select i1 %d, float %c, float 1.0
+  %f = fptrunc float %e to half
+  ret half %f
+}
+
 declare float @llvm.fabs.f32(float) nounwind readonly