[PATCH] D145326: [InstCombine] Transform `(shift X, Or(Y, BitWidth-1))` -> `(shift X,BitWidth-1)`

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n updated this revision to Diff 502466.
goldstein.w.n added a comment.

Use `replaceOperand` to propegate exact flag + simplifiy logic


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D145326/new/

https://reviews.llvm.org/D145326

Files:
  llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
  llvm/test/Transforms/InstCombine/shift.ll


Index: llvm/test/Transforms/InstCombine/shift.ll
===================================================================
--- llvm/test/Transforms/InstCombine/shift.ll
+++ llvm/test/Transforms/InstCombine/shift.ll
@@ -2067,3 +2067,48 @@
   %shl = shl i32 2, %tz
   ret i32 %shl
 }
+
+; shift (X, amt | bitwidth - 1) -> shift (X, bitwidth - 1)
+define i6 @shl_or7_eq_shl7(i6 %x, i6 %c) {
+; CHECK-LABEL: @shl_or7_eq_shl7(
+; CHECK-NEXT:    [[Y:%.*]] = shl nsw i6 [[X:%.*]], 5
+; CHECK-NEXT:    ret i6 [[Y]]
+;
+  %amt = or i6 %c, 5
+  ;; nsw not needed for transform, just check that we propegate.
+  %y = shl nsw i6 %x, %amt
+  ret i6 %y
+}
+
+define <2 x i8> @lshr_vec_or7_eq_shl7(<2 x i8> %x, <2 x i8> %c) {
+; CHECK-LABEL: @lshr_vec_or7_eq_shl7(
+; CHECK-NEXT:    [[Y:%.*]] = lshr exact <2 x i8> [[X:%.*]], <i8 7, i8 7>
+; CHECK-NEXT:    ret <2 x i8> [[Y]]
+;
+  %amt = or <2 x i8> %c, <i8 7, i8 7>
+  ;; exact not needed for transform, just check that we propegate.
+  %y = lshr exact <2 x i8> %x, %amt
+  ret <2 x i8> %y
+}
+
+define <2 x i8> @ashr_vec_or7_eq_ashr7(<2 x i8> %x, <2 x i8> %c) {
+; CHECK-LABEL: @ashr_vec_or7_eq_ashr7(
+; CHECK-NEXT:    [[Y:%.*]] = ashr <2 x i8> [[X:%.*]], <i8 7, i8 7>
+; CHECK-NEXT:    ret <2 x i8> [[Y]]
+;
+  %amt = or <2 x i8> %c, <i8 7, i8 7>
+  %y = ashr <2 x i8> %x, %amt
+  ret <2 x i8> %y
+}
+
+; Negative test not bitwidth - 1
+define <2 x i8> @ashr_vec_or6_fail(<2 x i8> %x, <2 x i8> %c) {
+; CHECK-LABEL: @ashr_vec_or6_fail(
+; CHECK-NEXT:    [[AMT:%.*]] = or <2 x i8> [[C:%.*]], <i8 6, i8 6>
+; CHECK-NEXT:    [[Y:%.*]] = ashr <2 x i8> [[X:%.*]], [[AMT]]
+; CHECK-NEXT:    ret <2 x i8> [[Y]]
+;
+  %amt = or <2 x i8> %c, <i8 6, i8 6>
+  %y = ashr <2 x i8> %x, %amt
+  ret <2 x i8> %y
+}
Index: llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -483,6 +483,9 @@
   if (Instruction *Logic = foldShiftOfShiftedBinOp(I, Builder))
     return Logic;
 
+  if (match(Op1, m_Or(m_Value(), m_SpecificInt(BitWidth - 1))))
+    return replaceOperand(I, 1, ConstantInt::get(Ty, BitWidth - 1));
+
   return nullptr;
 }
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D145326.502466.patch
Type: text/x-patch
Size: 2245 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20230305/5f7a487a/attachment.bin>