InstCombine: fold (sitofp (zext x)) to (uitofp x)

This is okay because the zext guarantees the high bit is zero,
and so the value is unsigned.

llvm-svn: 235364

2 files changed, 13 insertions, 1 deletions

diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 1c831ccd289..b2898429239 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -2118,7 +2118,7 @@ unsigned CastInst::isEliminableCastPair(
     // N  X  X  U  S  F  F  N  X  N  2  V  V   |
     // C  T  T  I  I  P  P  C  T  T  P  T  T  -+
     {  1, 0, 0,99,99, 0, 0,99,99,99, 0, 3, 0}, // Trunc         -+
-    {  8, 1, 9,99,99, 2, 0,99,99,99, 2, 3, 0}, // ZExt           |
+    {  8, 1, 9,99,99, 2,17,99,99,99, 2, 3, 0}, // ZExt           |
     {  8, 0, 1,99,99, 0, 2,99,99,99, 0, 3, 0}, // SExt           |
     {  0, 0, 0,99,99, 0, 0,99,99,99, 0, 3, 0}, // FPToUI         |
     {  0, 0, 0,99,99, 0, 0,99,99,99, 0, 3, 0}, // FPToSI         |
@@ -2285,6 +2285,9 @@ unsigned CastInst::isEliminableCastPair(
         "Illegal bitcast, ptrtoint sequence!");
       // Allowed, use second cast's opcode
       return secondOp;
+    case 17:
+      // (sitofp (zext x)) -> (uitofp x)
+      return Instruction::UIToFP;
     case 99: 
       // Cast combination can't happen (error in input). This is for all cases
       // where the MidTy is not the same for the two cast instructions.
diff --git a/llvm/test/Transforms/InstCombine/cast.ll b/llvm/test/Transforms/InstCombine/cast.ll
index c96140e2197..d4356d9364b 100644
--- a/llvm/test/Transforms/InstCombine/cast.ll
+++ b/llvm/test/Transforms/InstCombine/cast.ll
@@ -1104,3 +1104,12 @@ define i32 @PR21388(i32* %v) {
 ; CHECK-NEXT: %[[sext:.*]] = sext i1 %[[icmp]] to i32
 ; CHECK-NEXT: ret i32 %[[sext]]
 }
+
+define float @sitofp_zext(i16 %a) {
+; CHECK-LABEL: @sitofp_zext(
+; CHECK-NEXT: %[[sitofp:.*]] = uitofp i16 %a to float
+; CHECK-NEXT: ret float %[[sitofp]]
+  %zext = zext i16 %a to i32
+  %sitofp = sitofp i32 %zext to float
+  ret float %sitofp
+}