[InstCombine] C0 shift (X add nuw C) --> (C0 shift C) shift X

With 'nuw' we can convert the increment of the shift amount
into a pre-shift (constant fold) of the shifted constant:
https://alive2.llvm.org/ce/z/FkTyR2

Fixes issue #41976

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp

@@ -399,13 +399,14 @@ Instruction *InstCombinerImpl::commonShiftTransforms(BinaryOperator &I) {
           reassociateShiftAmtsOfTwoSameDirectionShifts(&I, SQ)))
     return NewShift;
 
-  // (C1 shift (A add C2)) -> (C1 shift C2) shift A)
-  // iff A and C2 are both positive.
+  // C0 shift (A add C) -> (C0 shift C) shift A
+  // iff A and C2 are both positive or the add has 'nuw'.
   Value *A;
   Constant *C;
   if (match(Op0, m_Constant()) && match(Op1, m_Add(m_Value(A), m_Constant(C))))
-    if (isKnownNonNegative(A, DL, 0, &AC, &I, &DT) &&
-        isKnownNonNegative(C, DL, 0, &AC, &I, &DT))
+    if (cast<BinaryOperator>(Op1)->hasNoUnsignedWrap() ||
+        (isKnownNonNegative(A, DL, 0, &AC, &I, &DT) &&
+         isKnownNonNegative(C, DL, 0, &AC, &I, &DT)))
       return BinaryOperator::Create(
           I.getOpcode(), Builder.CreateBinOp(I.getOpcode(), Op0, C), A);
 

llvm/test/Transforms/InstCombine/shift-add.ll

@@ -123,39 +123,42 @@ define <4 x i32> @lshr_C1_add_A_C2_v4i32_splat(i16 %I) {
 
 define i32 @shl_add_nuw(i32 %x) {
 ; CHECK-LABEL: @shl_add_nuw(
-; CHECK-NEXT:    [[A:%.*]] = add nuw i32 [[X:%.*]], 5
-; CHECK-NEXT:    [[R:%.*]] = shl i32 6, [[A]]
+; CHECK-NEXT:    [[R:%.*]] = shl i32 192, [[X:%.*]]
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %a = add nuw i32 %x, 5
   %r = shl i32 6, %a
   ret i32 %r
 }
 
+; vectors with arbitrary constants work too
+
 define <2 x i12> @lshr_add_nuw(<2 x i12> %x) {
 ; CHECK-LABEL: @lshr_add_nuw(
-; CHECK-NEXT:    [[A:%.*]] = add nuw <2 x i12> [[X:%.*]], <i12 5, i12 1>
-; CHECK-NEXT:    [[R:%.*]] = lshr <2 x i12> <i12 6, i12 42>, [[A]]
+; CHECK-NEXT:    [[R:%.*]] = lshr <2 x i12> <i12 0, i12 21>, [[X:%.*]]
 ; CHECK-NEXT:    ret <2 x i12> [[R]]
 ;
   %a = add nuw <2 x i12> %x, <i12 5, i12 1>
   %r = lshr <2 x i12> <i12 6, i12 42>, %a
   ret <2 x i12> %r
 }
 
+; extra use is ok and in this case the result can be simplified to a constant
+
 define i32 @ashr_add_nuw(i32 %x, i32* %p) {
 ; CHECK-LABEL: @ashr_add_nuw(
 ; CHECK-NEXT:    [[A:%.*]] = add nuw i32 [[X:%.*]], 5
 ; CHECK-NEXT:    store i32 [[A]], i32* [[P:%.*]], align 4
-; CHECK-NEXT:    [[R:%.*]] = ashr i32 -6, [[A]]
-; CHECK-NEXT:    ret i32 [[R]]
+; CHECK-NEXT:    ret i32 -1
 ;
   %a = add nuw i32 %x, 5
   store i32 %a, i32* %p
   %r = ashr i32 -6, %a
   ret i32 %r
 }
 
+; negative test - must have 'nuw'
+
 define i32 @shl_add_nsw(i32 %x) {
 ; CHECK-LABEL: @shl_add_nsw(
 ; CHECK-NEXT:    [[A:%.*]] = add nsw i32 [[X:%.*]], 5