[llvm] [RISCV][GISEL] Fix legalization for G_MERGE/UNMERGE_VALUES (PR #75619)

[Craig Topper via llvm-commits]

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@@ -85,14 +85,16 @@ RISCVLegalizerInfo::RISCVLegalizerInfo(const RISCVSubtarget &ST)
 
   // Merge/Unmerge
   for (unsigned Op : {G_MERGE_VALUES, G_UNMERGE_VALUES}) {
-    unsigned BigTyIdx = Op == G_MERGE_VALUES ? 0 : 1;
-    unsigned LitTyIdx = Op == G_MERGE_VALUES ? 1 : 0;
     auto &MergeUnmergeActions = getActionDefinitionsBuilder(Op);
     if (XLen == 32 && ST.hasStdExtD()) {
-      LLT IdxZeroTy = G_MERGE_VALUES ? s64 : s32;
-      LLT IdxOneTy = G_MERGE_VALUES ? s32 : s64;
-      MergeUnmergeActions.legalFor({IdxZeroTy, IdxOneTy});
+      // Only need to legalize on IdxZeroTy since there is no way to construct
+      // a valid MERGE/UNMERGE where the types don't combine/decompose as
+      // expected.
+      LLT IdxZeroTy = Op == G_MERGE_VALUES ? s64 : s32;
+      MergeUnmergeActions.legalFor({IdxZeroTy});
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topperc wrote:

> I have updated the code to do this, but I suppose there could be a case where you want to mark a type index as legal, regardless of what the other type index is.

You couldn't make type index 1 legal and ignore type index 0 though. So it would be inconsistent if that was the intent.

https://github.com/llvm/llvm-project/pull/75619