[GlobalISel] Combine G_MERGE_VALUES of x and zero (#116283)

into zext x

LegalizerHelper has two padding strategies: undef or zero.

see LegalizerHelper:273
see LegalizerHelper:315

This PR is about zero sugar and Coke Zero.

; CHECK-NEXT: [[MV2:%[0-9]+]]:_(s64) = G_MERGE_VALUES %a(s32),
[[C]](s32)

Please continue padding merge values.

// %bits_8_15:(s8) = G_CONSTANT i8 0
// %0:(s16) = G_MERGE_VALUES %bits_0_7:(s8), %bits_8_15:(s8)

%bits_8_15 is defined by zero. For optimization, we pick zext.

// %0:_(s16) = G_ZEXT %bits_0_7:(s8)

The upper bits of %0 are zero and the lower bits come from %bits_0_7.

Author: Thorsten Schütt

llvm/include/llvm/CodeGen/GlobalISel/CombinerHelper.h

@@ -936,6 +936,9 @@ class CombinerHelper {
   // merge_values(_, undef) -> anyext
   bool matchMergeXAndUndef(const MachineInstr &MI, BuildFnTy &MatchInfo);
 
+  // merge_values(_, zero) -> zext
+  bool matchMergeXAndZero(const MachineInstr &MI, BuildFnTy &MatchInfo);
+
 private:
   /// Checks for legality of an indexed variant of \p LdSt.
   bool isIndexedLoadStoreLegal(GLoadStore &LdSt) const;

llvm/include/llvm/Target/GlobalISel/Combine.td

@@ -865,6 +865,14 @@ def merge_of_x_and_undef : GICombineRule <
          [{ return Helper.matchMergeXAndUndef(*${MI}, ${matchinfo}); }]),
   (apply [{ Helper.applyBuildFn(*${MI}, ${matchinfo}); }])>;
 
+/// Transform merge_x_zero -> zext.
+def merge_of_x_and_zero : GICombineRule <
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (G_CONSTANT $zero, 0),
+         (G_MERGE_VALUES $root, $x, $zero):$MI,
+         [{ return Helper.matchMergeXAndZero(*${MI}, ${matchinfo}); }]),
+  (apply [{ Helper.applyBuildFn(*${MI}, ${matchinfo}); }])>;
+
 def merge_combines: GICombineGroup<[
   unmerge_anyext_build_vector,
   unmerge_merge,
@@ -873,7 +881,8 @@ def merge_combines: GICombineGroup<[
   unmerge_undef,
   unmerge_dead_to_trunc,
   unmerge_zext_to_zext,
-  merge_of_x_and_undef
+  merge_of_x_and_undef,
+  merge_of_x_and_zero
 ]>;
 
 // Under certain conditions, transform:

llvm/lib/CodeGen/GlobalISel/CombinerHelperArtifacts.cpp

@@ -56,3 +56,31 @@ bool CombinerHelper::matchMergeXAndUndef(const MachineInstr &MI,
   };
   return true;
 }
+
+bool CombinerHelper::matchMergeXAndZero(const MachineInstr &MI,
+                                        BuildFnTy &MatchInfo) {
+  const GMerge *Merge = cast<GMerge>(&MI);
+
+  Register Dst = Merge->getReg(0);
+  LLT DstTy = MRI.getType(Dst);
+  LLT SrcTy = MRI.getType(Merge->getSourceReg(0));
+
+  // No multi-use check. It is a constant.
+
+  //
+  //   %bits_8_15:_(s8) = G_CONSTANT i8 0
+  //   %0:_(s16) = G_MERGE_VALUES %bits_0_7:(s8), %bits_8_15:(s8)
+  //
+  // ->
+  //
+  //   %0:_(s16) = G_ZEXT %bits_0_7:(s8)
+  //
+
+  if (!isLegalOrBeforeLegalizer({TargetOpcode::G_ZEXT, {DstTy, SrcTy}}))
+    return false;
+
+  MatchInfo = [=](MachineIRBuilder &B) {
+    B.buildZExt(Dst, Merge->getSourceReg(0));
+  };
+  return true;
+}

llvm/test/CodeGen/AArch64/GlobalISel/combine-unmerge.mir

@@ -567,3 +567,49 @@ body:             |
     $q0 = COPY %me(s128)
     $x0 = COPY %def(s64)
 ...
+# Check that we zext the merge
+---
+name:            test_merge_zero
+body:             |
+  bb.1:
+    ; CHECK-LABEL: name: test_merge_zero
+    ; CHECK: %opaque:_(s64) = COPY $x0
+    ; CHECK-NEXT: %me:_(s128) = G_ZEXT %opaque(s64)
+    ; CHECK-NEXT: $q0 = COPY %me(s128)
+    %opaque:_(s64) = COPY $x0
+    %def:_(s64) = G_CONSTANT i64 0
+    %me:_(s128) = G_MERGE_VALUES %opaque(s64), %def
+    $q0 = COPY %me(s128)
+...
+# Check that we still zext the merge, multi-use
+---
+name:            test_merge_zero_multi_use
+body:             |
+  bb.1:
+    ; CHECK-LABEL: name: test_merge_zero_multi_use
+    ; CHECK: %opaque:_(s64) = COPY $x0
+    ; CHECK-NEXT: %def:_(s64) = G_CONSTANT i64 0
+    ; CHECK-NEXT: %me:_(s128) = G_ZEXT %opaque(s64)
+    ; CHECK-NEXT: $q0 = COPY %me(s128)
+    ; CHECK-NEXT: $x0 = COPY %def(s64)
+    %opaque:_(s64) = COPY $x0
+    %def:_(s64) = G_CONSTANT i64 0
+    %me:_(s128) = G_MERGE_VALUES %opaque(s64), %def
+    $q0 = COPY %me(s128)
+    $x0 = COPY %def(s64)
+...
+# Check that we don't zext the merge with one
+---
+name:            test_merge_one
+body:             |
+  bb.1:
+    ; CHECK-LABEL: name: test_merge_one
+    ; CHECK: %opaque:_(s64) = COPY $x0
+    ; CHECK-NEXT: %def:_(s64) = G_CONSTANT i64 1
+    ; CHECK-NEXT: %me:_(s128) = G_MERGE_VALUES %opaque(s64), %def(s64)
+    ; CHECK-NEXT: $q0 = COPY %me(s128)
+    %opaque:_(s64) = COPY $x0
+    %def:_(s64) = G_CONSTANT i64 1
+    %me:_(s128) = G_MERGE_VALUES %opaque(s64), %def
+    $q0 = COPY %me(s128)
+...