teach the truncation optimization that an entire chain of

computation can be truncated if it is fed by a sext/zext that doesn't
have to be exactly equal to the truncation result type.

llvm-svn: 112285

2 files changed, 26 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 1372a1891fa..89719bef09e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -396,6 +396,11 @@ static bool CanEvaluateTruncated(Value *V, const Type *Ty) {
   case Instruction::Trunc:
     // trunc(trunc(x)) -> trunc(x)
     return true;
+  case Instruction::ZExt:
+  case Instruction::SExt:
+    // trunc(ext(x)) -> ext(x) if the source type is smaller than the new dest
+    // trunc(ext(x)) -> trunc(x) if the source type is larger than the new dest
+    return true;
   case Instruction::Select: {
     SelectInst *SI = cast<SelectInst>(I);
     return CanEvaluateTruncated(SI->getTrueValue(), Ty) &&
diff --git a/llvm/test/Transforms/InstCombine/trunc.ll b/llvm/test/Transforms/InstCombine/trunc.ll
index 5cb34746676..f98bfd9236c 100644
--- a/llvm/test/Transforms/InstCombine/trunc.ll
+++ b/llvm/test/Transforms/InstCombine/trunc.ll
@@ -1,4 +1,5 @@
 ; RUN: opt < %s -instcombine -S | FileCheck %s
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
 
 ; Instcombine should be able to eliminate all of these ext casts.
 
@@ -76,7 +77,23 @@ define i92 @test7(i64 %A) {
   %D = trunc i128 %C to i92
   ret i92 %D
 ; CHECK: @test7
-; CHECK: %C = lshr i64 %A, 32
-; CHECK: %D = zext i64 %C to i92
-; CHECK: ret i92 %D
-}
\ No newline at end of file
+; CHECK: %B = zext i64 %A to i92
+; CHECK: %C = lshr i92 %B, 32
+; CHECK: ret i92 %C
+}
+
+define i64 @test8(i32 %A, i32 %B) {
+  %tmp38 = zext i32 %A to i128
+  %tmp32 = zext i32 %B to i128
+  %tmp33 = shl i128 %tmp32, 32
+  %ins35 = or i128 %tmp33, %tmp38
+  %tmp42 = trunc i128 %ins35 to i64
+  ret i64 %tmp42
+; CHECK: @test8
+; CHECK:   %tmp38 = zext i32 %A to i64
+; CHECK:   %tmp32 = zext i32 %B to i64
+; CHECK:   %tmp33 = shl i64 %tmp32, 32
+; CHECK:   %ins35 = or i64 %tmp33, %tmp38
+; CHECK:   ret i64 %ins35
+}
+