There is a trunc(lshr (zext A), Cst) optimization in InstCombineCasts that

removes cast by performing the lshr on smaller types. However, currently there
is no trunc(lshr (sext A), Cst) variant.
This patch add such optimization by transforming trunc(lshr (sext A), Cst)
to ashr A, Cst.

Differential Revision: http://reviews.llvm.org/D12520

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246997 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 72f0c1fc6a748bcc010e70d4b0919d8ba80c2ced..71ce74959c2d1e640456cc6e9691b11302bba052 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -499,6 +499,26 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
     return CastInst::CreateIntegerCast(Shift, CI.getType(), false);
   }
 
+  // Transform trunc(lshr (sext A), Cst) to ashr A, Cst to eliminate type
+  // conversion.
+  // It works because bits coming from sign extension have the same value as
+  // sign bit of the original value; performing ashr instead of lshr
+  // generates bits of the same value as the sign bit.
+  if (Src->hasOneUse() &&
+      match(Src, m_LShr(m_SExt(m_Value(A)), m_ConstantInt(Cst))) &&
+      cast<Instruction>(Src)->getOperand(0)->hasOneUse()) {
+    const unsigned ASize = A->getType()->getPrimitiveSizeInBits();
+    // This optimization can be only performed when zero bits generated by
+    // the original lshr aren't pulled into the value after truncation, so we
+    // can only shift by values smaller then the size of destination type (in
+    // bits).
+    if (Cst->getValue().ult(ASize)) {
+      Value *Shift = Builder->CreateAShr(A, Cst->getZExtValue());
+      Shift->takeName(Src);
+      return CastInst::CreateIntegerCast(Shift, CI.getType(), false);
+    }
+  }
+
   // Transform "trunc (and X, cst)" -> "and (trunc X), cst" so long as the dest
   // type isn't non-native.
   if (Src->hasOneUse() && isa<IntegerType>(Src->getType()) &&

diff --git a/test/Transforms/InstCombine/cast.ll b/test/Transforms/InstCombine/cast.ll
index 7fe54ef8469b784ac584071efab1e50f29bda13a..6edcbe2bea3d4cba95ab35fb9d044e95eb8de2bd 100644 (file)
--- a/test/Transforms/InstCombine/cast.ll
+++ b/test/Transforms/InstCombine/cast.ll
@@ -1062,6 +1062,43 @@ define i8 @test85(i32 %a) {
 ; CHECK: [[CST:%.*]] = trunc i32 [[SHR]] to i8
 }
 
+define i16 @test86(i16 %v) {
+  %a = sext i16 %v to i32
+  %s = ashr i32 %a, 4
+  %t = trunc i32 %s to i16
+  ret i16 %t
+
+; CHECK-LABEL: @test86(
+; CHECK:  [[ASHR:%.*]] = ashr i16 %v, 4
+; CHECK-NEXT: ret i16 [[ASHR]]
+}
+
+define i16 @test87(i16 %v) {
+  %c = sext i16 %v to i32
+  %m = mul nsw i32 %c, 16
+  %a = ashr i32 %m, 16
+  %t = trunc i32 %a to i16
+  ret i16 %t
+
+; CHECK-LABEL: @test87(
+; CHECK:  [[ASHR:%.*]] = ashr i16 %v, 12
+; CHECK-NEXT: ret i16 [[ASHR]]
+}
+
+define i16 @test88(i16 %v) {
+  %a = sext i16 %v to i32
+  %s = ashr i32 %a, 18
+  %t = trunc i32 %s to i16
+  ret i16 %t
+
+; Do not optimize to ashr i16 (shift by 18)
+; CHECK-LABEL: @test88(
+; CHECK: [[SEXT:%.*]] = sext i16 %v to i32
+; CHECK-NEXT: [[ASHR:%.*]] = ashr i32 [[SEXT]], 18
+; CHECK-NEXT: [[TRUNC:%.*]] = trunc i32 [[ASHR]] to i16
+; CHECK-NEXT: ret i16 [[TRUNC]]
+}
+
 ; Overflow on a float to int or int to float conversion is undefined (PR21130).
 
 define i8 @overflow_fptosi() {