[llvm] r247180 - Revert trunc(lshr (sext A), Cst) to ashr A, Cst

[David Majnemer via llvm-commits]

Author: majnemer
Date: Wed Sep  9 15:20:08 2015
New Revision: 247180

URL: http://llvm.org/viewvc/llvm-project?rev=247180&view=rev
Log:
Revert trunc(lshr (sext A), Cst) to ashr A, Cst

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

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
    llvm/trunk/test/Transforms/InstCombine/cast.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=247180&r1=247179&r2=247180&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Wed Sep  9 15:20:08 2015
@@ -498,26 +498,6 @@ Instruction *InstCombiner::visitTrunc(Tr
     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()) &&

Modified: llvm/trunk/test/Transforms/InstCombine/cast.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/cast.ll?rev=247180&r1=247179&r2=247180&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/cast.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/cast.ll Wed Sep  9 15:20:08 2015
@@ -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() {