Use a stable-sort when combining bases

[saugustine]

Summary

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 [e-mail] for finding the proper fix.

[efriedma]

LGTM

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,

We don't want the code generated by LLVM to depend on the way std::sort is implemented. Even if there isn't any functional difference, any sort of non-determinism makes it harder to understand bugs or performance issues. (Output changing based on the host C++ library isn't as bad as cases where it depends on ASLR randomization or something like that, but still.)

[MaskRay]

In D121870#3388248, @efriedma wrote:

LGTM

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,

We don't want the code generated by LLVM to depend on the way std::sort is implemented. Even if there isn't any functional difference, any sort of non-determinism makes it harder to understand bugs or performance issues. (Output changing based on the host C++ library isn't as bad as cases where it depends on ASLR randomization or something like that, but still.)

Agree. libc++ now provides -D_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY (D96946, or via _LIBCPP_DEBUG=1) to randomize std::sort.
Some build bots using libc++ may consider switching to this mode.

[clementval]

This change breaks some CI. Precheck even failed on this. Can you revert,

[craig.topper]

llvm/lib/Target/ARM/ARMISelLowering.cpp

15981

lambda needs to use const references that's probably what's causing the breakage.

llvm/lib/Target/ARM/ARMISelLowering.cpp

if (TryCombineBaseUpdate(Target, User, /*SimpleConstIncOnly=*/true, DCI))
       return SDValue();
     ++I;
   }
   BaseUpdates.resize(NumValidUpd);
 
   // 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(),
+  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();
   }
   return SDValue();
 }
 
 static SDValue PerformVLDCombine(SDNode *N,