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

[Nemanja Ivanovic 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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nemanjai wrote:

Actually, this is only true if we assume that the addend on the LO12 will be equal or greater than the addend on the HI20.
Ultimately, we need to ensure that the entire window of
`min(st_size, st_align)` bytes that contains the relocation is reachable from GP.

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