From df541601235ede978715cbdc24692e374a715442 Mon Sep 17 00:00:00 2001
From: Jakub Kuderski <jakub.kuderski@arm.com>
Date: Tue, 8 Sep 2015 10:03:17 +0000
Subject: [PATCH] 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
---
 .../InstCombine/InstCombineCasts.cpp          | 20 ++++++++++
 test/Transforms/InstCombine/cast.ll           | 37 +++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 72f0c1fc6a7..71ce74959c2 100644
--- 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 7fe54ef8469..6edcbe2bea3 100644
--- 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() {
-- 
2.34.1