[lld] [RISCV] Disable gp relaxation if part of object unreachable (PR #72655)

[Fangrui Song via llvm-commits]

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@@ -651,6 +651,20 @@ static void relaxHi20Lo12(const InputSection &sec, size_t i, uint64_t loc,
   if (!isInt<12>(r.sym->getVA(r.addend) - gp->getVA()))
     return;
 
+  // The symbol may be accessed in multiple pieces. We need to make sure that
+  // all of the possible accesses are relaxed or none are. This prevents
+  // relaxing the hi relocation and being unable to relax one of the low
+  // relocations. The compiler will only access multiple pieces of an object
+  // with low relocations on the memory op if the alignment allows it.
+  // Therefore it should suffice to check that the smaller of the alignment
+  // and size can be reached from GP.
+  uint32_t alignAdjust =
+      r.sym->getOutputSection() ? r.sym->getOutputSection()->addralign : 0;
+  alignAdjust = std::min<uint32_t>(alignAdjust, r.sym->getSize());
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MaskRay wrote:

If `addend < st_size`, it seems that this can be improved to `min(addralign, st_size-addend)`

https://github.com/llvm/llvm-project/pull/72655