[llvm] 8141726 - Use a stable-sort when combining bases

[Sterling Augustine via llvm-commits]

Author: Sterling Augustine
Date: 2022-03-17T09:02:13-07:00
New Revision: 81417261a15f46284f2613118120d7d6de2bc02d

URL: https://github.com/llvm/llvm-project/commit/81417261a15f46284f2613118120d7d6de2bc02d
DIFF: https://github.com/llvm/llvm-project/commit/81417261a15f46284f2613118120d7d6de2bc02d.diff

LOG: Use a stable-sort when combining bases

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.

Differential Revision: https://reviews.llvm.org/D121870

Added: 
    

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

Removed: 
    


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diff  --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 583b6b8afb5f3..f0e3b988642a7 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -15977,10 +15977,10 @@ static SDValue CombineBaseUpdate(SDNode *N,
   // 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();