[InstCombine] Added support for (X >>s C) << C --> X & (-1 << C)

[aaboud]

Summary

This patch is needed for D36679.

[spatel]

test/Transforms/InstCombine/shift.ll

225–235

This test doesn't change with this patch, does it? Demanded-bits should already get this. That's why my suggested test case included an extra use of the right-shifted value.

Also, I'm not sure if your intent was to purposely show a hole in utils/update_test_checks.py, but for the sake of human readability, please don't use variables that only differ in capitalization. :)

[aaboud]

Updated the test to a more relevant one.

[aaboud]

test/Transforms/InstCombine/shift.ll

225–235

Sorry, I missed the point that the "ashr" instruction was used twice on the test you suggested.
I updated the patch accordingly.

[spatel]

LGTM.

lib/Transforms/InstCombine/InstCombineShifts.cpp

if (match(Op1, m_APInt(ShAmtAPInt))) {
     Value *X;
     if (match(Op0, m_ZExt(m_Value(X)))) {
       unsigned SrcWidth = X->getType()->getScalarSizeInBits();
       if (ShAmt < SrcWidth &&
           MaskedValueIsZero(X, APInt::getHighBitsSet(SrcWidth, ShAmt), 0, &I))
         return new ZExtInst(Builder.CreateShl(X, ShAmt), Ty);
     }
 
-    // (X >>u C) << C --> X & (-1 << C)
-    if (match(Op0, m_LShr(m_Value(X), m_Specific(Op1)))) {
+    // (X >> C) << C --> X & (-1 << C)
+    if (match(Op0, m_Shr(m_Value(X), m_Specific(Op1)))) {
       APInt Mask(APInt::getHighBitsSet(BitWidth, BitWidth - ShAmt));
       return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, Mask));
     }
 
     // Be careful about hiding shl instructions behind bit masks. They are used
     // to represent multiplies by a constant, and it is important that simple
     // arithmetic expressions are still recognizable by scalar evolution.
     // The inexact versions are deferred to DAGCombine, so we don't hide shl

test/Transforms/InstCombine/shift.ll

 ; CHECK-NEXT:    [[B1:%.*]] = and i32 %A, -256
 ; CHECK-NEXT:    ret i32 [[B1]]
 ;
   %B = ashr i32 %A, 8
   %C = shl i32 %B, 8
   ret i32 %C
 }
 
+;; ((A | 0xC0000000) >> 8) << 8 === (A & 3FFFFF00) | 0xC0000000
+define i32 @test12a(i32 %A) {
+; CHECK-LABEL: @test12a(
+; CHECK-NEXT:    [[a:%.*]] = and i32 %A, 1073741568
+; CHECK-NEXT:    [[C:%.*]] = or i32 [[a]], -1073741824
+; CHECK-NEXT:    ret i32 [[C]]
+;
+  %a = or i32 %A, -1073741824
+  %B = ashr i32 %a, 8
+  %C = shl i32 %B, 8
+  ret i32 %C
+}
+
 ;; This transformation is deferred to DAGCombine:
 ;; (A >> 3) << 4 === (A & -8) * 2
 ;; The shl may be valuable to scalar evolution.
 define i8 @test13(i8 %A) {
 ; CHECK-LABEL: @test13(
 ; CHECK-NEXT:    [[A:%.*]] = mul i8 %A, 3
 ; CHECK-NEXT:    [[B1:%.*]] = lshr i8 [[A]], 3
 ; CHECK-NEXT:    [[C:%.*]] = shl i8 [[B1]], 4