[PATCH] D121870: Use a stable-sort when combining bases

[Sterling Augustine via Phabricator via llvm-commits]

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While experimenting with different algorithms for std::sort
I discovered that combine-vmovdrr.ll fails if this sort is not
stable.

I suspect that the test is too stringent in its check--the resultant
code looks functionally identical to me under both stable and unstable
sorting, but a generic fix is quite a bit more difficult to implement.

Thanks to scw at google.com for finding the proper fix.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D121870

Files:
  llvm/lib/Target/ARM/ARMISelLowering.cpp


Index: llvm/lib/Target/ARM/ARMISelLowering.cpp
===================================================================
--- llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -15977,10 +15977,10 @@
   // Try to fold with other users. Non-constant updates are considered
   // first, and constant updates are sorted to not break a sequence of
   // strided accesses (if there is any).
-  std::sort(BaseUpdates.begin(), BaseUpdates.end(),
-            [](BaseUpdateUser &LHS, BaseUpdateUser &RHS) {
-              return LHS.ConstInc < RHS.ConstInc;
-            });
+  std::stable_sort(BaseUpdates.begin(), BaseUpdates.end(),
+                   [](BaseUpdateUser &LHS, BaseUpdateUser &RHS) {
+                     return LHS.ConstInc < RHS.ConstInc;
+                   });
   for (BaseUpdateUser &User : BaseUpdates) {
     if (TryCombineBaseUpdate(Target, User, /*SimpleConstIncOnly=*/false, DCI))
       return SDValue();


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