[llvm] [GlobalISel] Combine (X >> C) << C to X & ((-1 >> C) << C) (PR #114821)

[Dávid Ferenc Szabó via llvm-commits]

================
@@ -365,6 +365,13 @@ def bitreverse_lshr : GICombineRule<
                                                     MRI.getType(${amt}.getReg())}}); }]),
   (apply (G_SHL $d, $val, $amt))>;
 
+def lsb_bits_clearing_by_shifts_to_and: GICombineRule<
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (G_LSHR $d, $src, $amt1),
+         (G_SHL $root, $d, $amt2):$root,
----------------
dfszabo wrote:

I am a bit suprised this is working for case where there is just a single G_CONSTANT. Now the only problem is that this pattern will not match the vector test case.
```diff
+++ b/llvm/include/llvm/Target/GlobalISel/Combine.td
@@ -367,7 +367,9 @@ def bitreverse_lshr : GICombineRule<
 
 def lsb_bits_clearing_by_shifts_to_and: GICombineRule<
   (defs root:$root, build_fn_matchinfo:$matchinfo),
-  (match (G_LSHR $d, $src, $amt1),
+  (match (G_CONSTANT $amt1, $imm1),
+         (G_CONSTANT $amt2, $imm2),
+         (G_LSHR $d, $src, $amt1),
          (G_SHL $root, $d, $amt2):$root,
          [{ return Helper.matchLsbClearByShifts(*${root}, ${matchinfo}); }]),
   (apply [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/combine-shifts-to-and.mir b/llvm/test/CodeGen/AArch64/GlobalISel/combine-shifts-to-and.mir
index 71236fb980e3..57c96f9fb9e1 100644
--- a/llvm/test/CodeGen/AArch64/GlobalISel/combine-shifts-to-and.mir
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/combine-shifts-to-and.mir
@@ -124,10 +124,11 @@ body:             |
     ; CHECK-NEXT: [[COPY:%[0-9]+]]:_(p0) = COPY $x0
     ; CHECK-NEXT: [[COPY1:%[0-9]+]]:_(s32) = COPY $w1
     ; CHECK-NEXT: %xvec:_(<4 x s32>) = G_BUILD_VECTOR [[COPY1]](s32), [[COPY1]](s32), [[COPY1]](s32), [[COPY1]](s32)
-    ; CHECK-NEXT: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 -4
-    ; CHECK-NEXT: [[BUILD_VECTOR:%[0-9]+]]:_(<4 x s32>) = G_BUILD_VECTOR [[C]](s32), [[C]](s32), [[C]](s32), [[C]](s32)
-    ; CHECK-NEXT: [[AND:%[0-9]+]]:_(<4 x s32>) = G_AND %xvec, [[BUILD_VECTOR]]
-    ; CHECK-NEXT: G_STORE [[AND]](<4 x s32>), [[COPY]](p0) :: (store (<4 x s32>))
+    ; CHECK-NEXT: [[C:%[0-9]+]]:_(s32) = G_CONSTANT i32 2
+    ; CHECK-NEXT: %veccst:_(<4 x s32>) = G_BUILD_VECTOR [[C]](s32), [[C]](s32), [[C]](s32), [[C]](s32)
+    ; CHECK-NEXT: [[LSHR:%[0-9]+]]:_(<4 x s32>) = G_LSHR %xvec, %veccst(<4 x s32>)
+    ; CHECK-NEXT: [[SHL:%[0-9]+]]:_(<4 x s32>) = G_SHL [[LSHR]], %veccst(<4 x s32>)
+    ; CHECK-NEXT: G_STORE [[SHL]](<4 x s32>), [[COPY]](p0) :: (store (<4 x s32>))
     ; CHECK-NEXT: RET_ReallyLR
     %0:_(p0) = COPY $x0
     %1:_(s32) = COPY $w1

```

https://github.com/llvm/llvm-project/pull/114821