[llvm] [GlobalISel] Add `combine` action for C++ combine rules (PR #135941)

[Matt Arsenault via llvm-commits]

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@@ -798,17 +798,45 @@ bool CombineRuleBuilder::parseAll() {
   if (!parseDefs(*RuleDef.getValueAsDag("Defs")))
     return false;
 
-  if (!Parser.parsePatternList(
-          *RuleDef.getValueAsDag("Match"),
-          [this](auto Pat) { return addMatchPattern(std::move(Pat)); }, "match",
-          (RuleDef.getName() + "_match").str()))
-    return false;
+  const DagInit &Act0 = *RuleDef.getValueAsDag("Action0");
+  const DagInit &Act1 = *RuleDef.getValueAsDag("Action1");
+
+  StringRef Act0Op = Act0.getOperatorAsDef(RuleDef.getLoc())->getName();
+  StringRef Act1Op = Act1.getOperatorAsDef(RuleDef.getLoc())->getName();
+
+  if (Act0Op == "match" && Act1Op == "apply") {
+    if (!Parser.parsePatternList(
+            Act0, [this](auto Pat) { return addMatchPattern(std::move(Pat)); },
+            "match", (RuleDef.getName() + "_match").str()))
+      return false;
+
+    if (!Parser.parsePatternList(
+            Act1, [this](auto Pat) { return addApplyPattern(std::move(Pat)); },
+            "apply", (RuleDef.getName() + "_apply").str()))
+      return false;
+
+  } else if (Act0Op == "combine" && Act1Op == "empty_action") {
+    // combine: everything is a "match" except C++ code which is an apply.
+    const auto AddCombinePat = [this](std::unique_ptr<Pattern> Pat) {
+      if (isa<CXXPattern>(Pat.get()))
+        return addApplyPattern(std::move(Pat));
+      return addMatchPattern(std::move(Pat));
+    };
 
-  if (!Parser.parsePatternList(
-          *RuleDef.getValueAsDag("Apply"),
-          [this](auto Pat) { return addApplyPattern(std::move(Pat)); }, "apply",
-          (RuleDef.getName() + "_apply").str()))
+    if (!Parser.parsePatternList(Act0, AddCombinePat, "combine",
+                                 (RuleDef.getName() + "_combine").str()))
+      return false;
+
+    if (MatchPats.empty() || ApplyPats.empty()) {
+      PrintError("'combine' action needs at least one pattern to match, and "
+                 "C++ code to apply");
+      return false;
+    }
+  } else {
+    PrintError("Expected both a 'match' and 'apply' action in combine rule, "
----------------
arsenm wrote:

Error messages should start with lowercase 

https://github.com/llvm/llvm-project/pull/135941