[AArch64] Fix register reversing crash on MSVC with 1 pair.

[ScottTodd]

This fixes the crash reported here: #79623 (comment) where this assert was hit:

#if _ITERATOR_DEBUG_LEVEL != 0
template <class _Ty>
constexpr void _Verify_range(const _Ty* const _First, const _Ty* const _Last) noexcept {
    // special case range verification for pointers
    _STL_VERIFY(_First <= _Last, "transposed pointer range");
}
#endif // _ITERATOR_DEBUG_LEVEL != 0

Info from the debugger on the test case that crashed:

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: Scott Todd (ScottTodd)

Changes

This fixes the crash reported here: #79623 (comment) where this assert was hit:

#if _ITERATOR_DEBUG_LEVEL != 0
template <class _Ty>
constexpr void _Verify_range(const _Ty* const _First, const _Ty* const _Last) noexcept {
    // special case range verification for pointers
    _STL_VERIFY(_First <= _Last, "transposed pointer range");
}
#endif // _ITERATOR_DEBUG_LEVEL != 0

Info from the debugger on the test case that crashed:

---

Full diff: https://github.com/llvm/llvm-project/pull/82392.diff

1 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64FrameLowering.cpp (+10-8)

diff --git a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
index c013bbe9926fef..6c8117cb978df5 100644
--- a/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
@@ -3115,14 +3115,16 @@ bool AArch64FrameLowering::restoreCalleeSavedRegisters(
   }
 
   // For performance reasons restore SVE register in increasing order
-  auto IsPPR = [](const RegPairInfo &c) { return c.Type == RegPairInfo::PPR; };
-  auto PPRBegin = std::find_if(RegPairs.begin(), RegPairs.end(), IsPPR);
-  auto PPREnd = std::find_if(RegPairs.rbegin(), RegPairs.rend(), IsPPR);
-  std::reverse(PPRBegin, PPREnd.base());
-  auto IsZPR = [](const RegPairInfo &c) { return c.Type == RegPairInfo::ZPR; };
-  auto ZPRBegin = std::find_if(RegPairs.begin(), RegPairs.end(), IsZPR);
-  auto ZPREnd = std::find_if(RegPairs.rbegin(), RegPairs.rend(), IsZPR);
-  std::reverse(ZPRBegin, ZPREnd.base());
+  if (RegPairs.size() > 1) {
+    auto IsPPR = [](const RegPairInfo &c) { return c.Type == RegPairInfo::PPR; };
+    auto PPRBegin = std::find_if(RegPairs.begin(), RegPairs.end(), IsPPR);
+    auto PPREnd = std::find_if(RegPairs.rbegin(), RegPairs.rend(), IsPPR);
+    std::reverse(PPRBegin, PPREnd.base());
+    auto IsZPR = [](const RegPairInfo &c) { return c.Type == RegPairInfo::ZPR; };
+    auto ZPRBegin = std::find_if(RegPairs.begin(), RegPairs.end(), IsZPR);
+    auto ZPREnd = std::find_if(RegPairs.rbegin(), RegPairs.rend(), IsZPR);
+    std::reverse(ZPRBegin, ZPREnd.base());
+  }
 
   for (const RegPairInfo &RPI : RegPairs) {
     unsigned Reg1 = RPI.Reg1;

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[momchil-velikov]

I suspect the issue is with dereferencing PPREnd.base() when PPREnd == RegPairs.rend() (same with ZPR).
And it can happen even if RegPairs.size() > 1, for example if there are no PPR pairs at all.
(it'll be a couple of hours before I get my hands on a Windows machine to confirm and/or investigate).

[ScottTodd]

I suspect the issue is with dereferencing PPREnd.base() when PPREnd == RegPairs.rend() (same with ZPR). And it can happen even if RegPairs.size() > 1, for example if there are no PPR pairs at all. (it'll be a couple of hours before I get my hands on a Windows machine to confirm and/or investigate).

Ah, interesting. Should we revert for now and then land with these cases fixed?

[ScottTodd]

Commits were reverted. LMK if I can help test other fixes - not sure I have enough context here to iterate on this patch.