[GlobalISel][Legalizer] add minScalarIf action

[stuij]

Summary

Ensure scalar is at least as wide as type, but only if the specified condition
is met.

[arsenm]

llvm/include/llvm/CodeGen/GlobalISel/LegalizerInfo.h

966

Should only use changeElementTo if this handles vectors, which it doesn't. Use changeTo or make minScalarOrEltIf

[stuij]

addressed review comment

[stuij]

llvm/include/llvm/CodeGen/GlobalISel/LegalizerInfo.h

966

thanks! done.

[paquette]

LGTM

llvm/include/llvm/CodeGen/GlobalISel/LegalizerInfo.h

#endif
   LegalizeRuleSet &minScalar(unsigned TypeIdx, const LLT Ty) {
     using namespace LegalityPredicates;
     using namespace LegalizeMutations;
     return actionIf(LegalizeAction::WidenScalar,
                     scalarNarrowerThan(TypeIdx, Ty.getSizeInBits()),
                     changeTo(typeIdx(TypeIdx), Ty));
   }
 
+  /// Ensure the scalar is at least as wide as Ty if condition is met.
+  LegalizeRuleSet &minScalarIf(LegalityPredicate Predicate, unsigned TypeIdx,
+                               const LLT Ty) {
+    using namespace LegalityPredicates;
+    using namespace LegalizeMutations;
+    return actionIf(
+        LegalizeAction::WidenScalar,
+        [=](const LegalityQuery &Query) {
+          const LLT QueryTy = Query.Types[TypeIdx];
+          return QueryTy.isScalar() &&
+                 QueryTy.getSizeInBits() < Ty.getSizeInBits() &&
+                 Predicate(Query);
+        },
+        changeTo(typeIdx(TypeIdx), Ty));
+  }
+
   /// Ensure the scalar is at most as wide as Ty.
   LegalizeRuleSet &maxScalarOrElt(unsigned TypeIdx, const LLT Ty) {
     using namespace LegalityPredicates;
     using namespace LegalizeMutations;
     return actionIf(LegalizeAction::NarrowScalar,
                     scalarOrEltWiderThan(TypeIdx, Ty.getScalarSizeInBits()),
                     changeElementTo(typeIdx(TypeIdx), Ty));
   }

llvm/unittests/CodeGen/GlobalISel/LegalizerInfoTest.cpp

LI.getActionDefinitionsBuilder(G_OR)
       .minScalar(0, s32);
     LegacyInfo.computeTables();
 
     // Only handle scalars, ignore vectors.
     EXPECT_ACTION(WidenScalar, 0, s32, LegalityQuery(G_OR, {s16}));
     EXPECT_ACTION(Unsupported, 0, LLT(), LegalityQuery(G_OR, {v2s16}));
   }
 
+  // Test minScalarIf
+  {
+    bool IfCond = true;
+    LegalizerInfo LI;
+    auto &LegacyInfo = LI.getLegacyLegalizerInfo();
+    LI.getActionDefinitionsBuilder(G_OR)
+      .legalFor({s32})
+      .minScalarIf([&](const LegalityQuery &Query) {
+                     return IfCond;
+                   }, 0, s32);
+    LegacyInfo.computeTables();
+
+    // Only handle scalars, ignore vectors.
+    EXPECT_ACTION(WidenScalar, 0, s32, LegalityQuery(G_OR, {s16}));
+    EXPECT_ACTION(Unsupported, 0, LLT(), LegalityQuery(G_OR, {v2s16}));
+
+    IfCond = false;
+    EXPECT_ACTION(Unsupported, 0, LLT(), LegalityQuery(G_OR, {s16}));
+    EXPECT_ACTION(Unsupported, 0, LLT(), LegalityQuery(G_OR, {v2s16}));
+  }
+
   // Test maxScalar
   {
     LegalizerInfo LI;
     auto &LegacyInfo = LI.getLegacyLegalizerInfo();
     LI.getActionDefinitionsBuilder(G_AND)
       .legalFor({s16})
       .maxScalar(0, s16);
     LegacyInfo.computeTables();