From 26122f9e6ace33d7ba57c10aeb5bcda6e18321be Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 9 Sep 2015 20:20:08 +0000
Subject: [PATCH] Revert trunc(lshr (sext A), Cst) to ashr A, Cst

This reverts commit r246997, it introduced a regression (PR24763).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247180 91177308-0d34-0410-b5e6-96231b3b80d8
---
 .../InstCombine/InstCombineCasts.cpp          | 20 ----------
 test/Transforms/InstCombine/cast.ll           | 37 -------------------
 2 files changed, 57 deletions(-)

diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index fabf80b3fd0..768616f4233 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -498,26 +498,6 @@ 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 6edcbe2bea3..7fe54ef8469 100644
--- a/test/Transforms/InstCombine/cast.ll
+++ b/test/Transforms/InstCombine/cast.ll
@@ -1062,43 +1062,6 @@ 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