now that the cost model has changed, we can always consider

elimination of a sign extend to be a win, which simplifies 
the client of CanEvaluateSExtd, and allows us to eliminate
more casts (examples taken from real code).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93109 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 60 insertions, 27 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index b0d017e062..13f60716f2 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -919,34 +919,25 @@ Instruction *InstCombiner::visitSExt(SExtInst &CI) {
     if (NumBitsSExt == 0)
       return 0;
     
+    // Okay, we can transform this!  Insert the new expression now.
+    DEBUG(dbgs() << "ICE: EvaluateInDifferentType converting expression type"
+          " to avoid sign extend: " << CI);
+    Value *Res = EvaluateInDifferentType(Src, DestTy, true);
+    assert(Res->getType() == DestTy);
+
     uint32_t SrcBitSize = SrcTy->getScalarSizeInBits();
     uint32_t DestBitSize = DestTy->getScalarSizeInBits();
+
+    // If the high bits are already filled with sign bit, just replace this
+    // cast with the result.
+    if (NumBitsSExt > DestBitSize - SrcBitSize ||
+        ComputeNumSignBits(Res) > DestBitSize - SrcBitSize)
+      return ReplaceInstUsesWith(CI, Res);
     
-    // Because this is a sign extension, we can always transform it by inserting
-    // two new shifts (to do the extension).  However, this is only profitable
-    // if we've eliminated two or more casts from the input.  If we know the
-    // result will be sign-extended enough to not require these shifts, we can
-    // always do the transformation.
-    if (NumCastsRemoved >= 2 ||
-        NumBitsSExt > DestBitSize-SrcBitSize) {
-      
-      // Okay, we can transform this!  Insert the new expression now.
-      DEBUG(dbgs() << "ICE: EvaluateInDifferentType converting expression type"
-            " to avoid sign extend: " << CI);
-      Value *Res = EvaluateInDifferentType(Src, DestTy, true);
-      assert(Res->getType() == DestTy);
-      
-      // If the high bits are already filled with sign bit, just replace this
-      // cast with the result.
-      if (NumBitsSExt > DestBitSize - SrcBitSize ||
-          ComputeNumSignBits(Res) > DestBitSize - SrcBitSize)
-        return ReplaceInstUsesWith(CI, Res);
-      
-      // We need to emit a shl + ashr to do the sign extend.
-      Value *ShAmt = ConstantInt::get(DestTy, DestBitSize-SrcBitSize);
-      return BinaryOperator::CreateAShr(Builder->CreateShl(Res, ShAmt, "sext"),
-                                        ShAmt);
-    }
+    // We need to emit a shl + ashr to do the sign extend.
+    Value *ShAmt = ConstantInt::get(DestTy, DestBitSize-SrcBitSize);
+    return BinaryOperator::CreateAShr(Builder->CreateShl(Res, ShAmt, "sext"),
+                                      ShAmt);
   }
 
   // If the input is a shl/ashr pair of a same constant, then this is a sign
diff --git a/test/Transforms/InstCombine/cast.ll b/test/Transforms/InstCombine/cast.ll
index de7f8c11f2..4f0475ac59 100644
--- a/test/Transforms/InstCombine/cast.ll
+++ b/test/Transforms/InstCombine/cast.ll
@@ -185,8 +185,8 @@ define i32 @test22(i32 %X) {
         %c2 = sext i8 %c1 to i32                ; <i32> [#uses=1]
         %RV = shl i32 %c2, 24           ; <i32> [#uses=1]
         ret i32 %RV
-; CHECK: %RV = shl i32 %X, 24
-; CHECK: ret i32 %RV
+; CHECK: shl i32 %X, 24
+; CHECK-NEXT: ret i32
 }
 
 define i32 @test23(i32 %X) {
@@ -457,3 +457,45 @@ define i64 @test48(i8 %A, i8 %a) {
 ; CHECK-NEXT: ret i64 %D
 }
 
+define i64 @test49(i64 %A) {
+ %B = trunc i64 %A to i32
+ %C = or i32 %B, 1
+ %D = sext i32 %C to i64 
+ ret i64 %D
+; CHECK: @test49
+; CHECK-NEXT: %C = shl i64 %A, 32
+; CHECK-NEXT: ashr i64 %C, 32
+; CHECK-NEXT: %D = or i64 {{.*}}, 1
+; CHECK-NEXT: ret i64 %D
+}
+
+define i64 @test50(i64 %A) {
+  %a = lshr i64 %A, 2
+  %B = trunc i64 %a to i32
+  %D = add i32 %B, -1
+  %E = sext i32 %D to i64
+  ret i64 %E
+; CHECK: @test50
+; CHECK-NEXT: shl i64 %A, 30
+; CHECK-NEXT: add i64 {{.*}}, -4294967296
+; CHECK-NEXT: %E = ashr i64 {{.*}}, 32
+; CHECK-NEXT: ret i64 %E
+}
+
+define i64 @test51(i64 %A, i1 %cond) {
+  %B = trunc i64 %A to i32
+  %C = and i32 %B, -2
+  %D = or i32 %B, 1
+  %E = select i1 %cond, i32 %C, i32 %D
+  %F = sext i32 %E to i64
+  ret i64 %F
+; CHECK: @test51
+; CHECK-NEXT: %C = and i64 %A, 4294967294
+; CHECK-NEXT: %D = or i64 %A, 1
+; CHECK-NEXT: %E = select i1 %cond, i64 %C, i64 %D
+; CHECK-NEXT: %sext = shl i64 %E, 32
+; CHECK-NEXT: %F = ashr i64 %sext, 32
+; CHECK-NEXT: ret i64 %F
+
+}
+