Relax PCHi20Lo12.

Support relaxation optimization for two types of code sequences.
```
From:
  pcalau12i $a0, %pc_hi20(sym)
    R_LARCH_PCALA_HI20, R_LARCH_RELAX
  addi.w/d $a0, $a0, %pc_lo12(sym)
    R_LARCH_PCALA_LO12, R_LARCH_RELAX
To:
  pcaddi $a0, %pc_lo12(sym)
    R_LARCH_PCREL20_S2

From:
  pcalau12i $a0, %got_pc_hi20(sym_got)
    R_LARCH_GOT_PC_HI20, R_LARCH_RELAX
  ld.w/d $a0, $a0, %got_pc_lo12(sym_got)
    R_LARCH_GOT_PC_LO12, R_LARCH_RELAX
To:
  pcaddi $a0, %got_pc_hi20(sym_got)
    R_LARCH_PCREL20_S2
```

Author: ylzsx

lld/ELF/Arch/LoongArch.cpp

@@ -53,6 +53,7 @@ enum Op {
   ADDI_W = 0x02800000,
   ADDI_D = 0x02c00000,
   ANDI = 0x03400000,
+  PCADDI = 0x18000000,
   PCADDU12I = 0x1c000000,
   LD_W = 0x28800000,
   LD_D = 0x28c00000,
@@ -131,6 +132,8 @@ static uint32_t extractBits(uint64_t v, uint32_t begin, uint32_t end) {
   return begin == 63 ? v >> end : (v & ((1ULL << (begin + 1)) - 1)) >> end;
 }
 
+static uint32_t getD5(uint64_t v) { return extractBits(v, 4, 0); }
+
 static uint32_t setD5k16(uint32_t insn, uint32_t imm) {
   uint32_t immLo = extractBits(imm, 15, 0);
   uint32_t immHi = extractBits(imm, 20, 16);
@@ -743,6 +746,84 @@ void LoongArch::relocate(uint8_t *loc, const Relocation &rel,
   }
 }
 
+static bool relaxable(ArrayRef<Relocation> relocs, size_t i) {
+  return i + 1 < relocs.size() && relocs[i + 1].type == R_LARCH_RELAX;
+}
+
+static bool isPairRelaxable(ArrayRef<Relocation> relocs, size_t i) {
+  return relaxable(relocs, i) && relaxable(relocs, i + 2) &&
+         relocs[i].offset + 4 == relocs[i + 2].offset;
+}
+
+// Relax code sequence.
+// From:
+//   pcalau12i $a0, %pc_hi20(sym)
+//   addi.w/d $a0, $a0, %pc_lo12(sym)
+// To:
+//   pcaddi $a0, %pc_lo12(sym)
+//
+// From:
+//   pcalau12i $a0, %got_pc_hi20(sym_got)
+//   ld.w/d $a0, $a0, %got_pc_lo12(sym_got)
+// To:
+//   pcaddi $a0, %got_pc_hi20(sym_got)
+static void relaxPCHi20Lo12(Ctx &ctx, const InputSection &sec, size_t i,
+                            uint64_t loc, Relocation &rHi20, Relocation &rLo12,
+                            uint32_t &remove) {
+  // check if the relocations are relaxable sequences.
+  if (!((rHi20.type == R_LARCH_PCALA_HI20 &&
+         rLo12.type == R_LARCH_PCALA_LO12) ||
+        (rHi20.type == R_LARCH_GOT_PC_HI20 &&
+         rLo12.type == R_LARCH_GOT_PC_LO12)))
+    return;
+
+  // GOT references to absolute symbols can't be relaxed to use pcaddi in
+  // position-independent code, because these instructions produce a relative
+  // address.
+  // Meanwhile skip undefined, preemptible and STT_GNU_IFUNC symbols, because
+  // these symbols may be resolve in runtime.
+  if (rHi20.type == R_LARCH_GOT_PC_HI20 &&
+      (!rHi20.sym->isDefined() || rHi20.sym->isPreemptible ||
+       rHi20.sym->isGnuIFunc() ||
+       (ctx.arg.isPic && !cast<Defined>(*rHi20.sym).section)))
+    return;
+
+  uint64_t symBase = 0;
+  if (rHi20.expr == RE_LOONGARCH_PLT_PAGE_PC)
+    symBase = rHi20.sym->getPltVA(ctx);
+  else if (rHi20.expr == RE_LOONGARCH_PAGE_PC ||
+           rHi20.expr == RE_LOONGARCH_GOT_PAGE_PC)
+    symBase = rHi20.sym->getVA(ctx);
+  else {
+    Err(ctx) << getErrorLoc(ctx, (const uint8_t *)loc) << "unknown expr ("
+             << rHi20.expr << ") against symbol " << rHi20.sym
+             << "in relaxPCHi20Lo12";
+    return;
+  }
+  const uint64_t symLocal = symBase + rHi20.addend;
+
+  const int64_t distance = symLocal - loc;
+  // Check if the distance aligns 4 bytes or exceeds the range of pcaddi.
+  if ((distance & 0x3) != 0 || !isInt<22>(distance))
+    return;
+
+  // Note: If we can ensure that the .o files generated by LLVM only contain
+  // relaxable instruction sequences with R_LARCH_RELAX, then we do not need to
+  // decode instructions. The relaxable instruction sequences imply the
+  // following constraints:
+  // * For relocation pairs related to got_pc, the opcodes of instructions
+  // must be pcalau12i + ld.w/d. In other cases, the opcodes must be pcalau12i +
+  // addi.w/d.
+  // * The destination register of pcalau12i is guaranteed to be used only by
+  // the immediately following instruction.
+  const uint32_t nextInsn = read32le(sec.content().data() + rLo12.offset);
+
+  sec.relaxAux->relocTypes[i] = R_LARCH_RELAX;
+  sec.relaxAux->relocTypes[i + 2] = R_LARCH_PCREL20_S2;
+  sec.relaxAux->writes.push_back(insn(PCADDI, getD5(nextInsn), 0, 0));
+  remove = 4;
+}
+
 static bool relax(Ctx &ctx, InputSection &sec) {
   const uint64_t secAddr = sec.getVA();
   const MutableArrayRef<Relocation> relocs = sec.relocs();
@@ -781,6 +862,12 @@ static bool relax(Ctx &ctx, InputSection &sec) {
       }
       break;
     }
+    case R_LARCH_PCALA_HI20:
+    case R_LARCH_GOT_PC_HI20:
+      // The overflow check for i+2 will be carried out in isPairRelaxable.
+      if (isPairRelaxable(relocs, i))
+        relaxPCHi20Lo12(ctx, sec, i, loc, r, relocs[i + 2], remove);
+      break;
     }
 
     // For all anchors whose offsets are <= r.offset, they are preceded by
@@ -851,6 +938,7 @@ void LoongArch::finalizeRelax(int passes) const {
       MutableArrayRef<Relocation> rels = sec->relocs();
       ArrayRef<uint8_t> old = sec->content();
       size_t newSize = old.size() - aux.relocDeltas[rels.size() - 1];
+      size_t writesIdx = 0;
       uint8_t *p = ctx.bAlloc.Allocate<uint8_t>(newSize);
       uint64_t offset = 0;
       int64_t delta = 0;
@@ -867,11 +955,29 @@ void LoongArch::finalizeRelax(int passes) const {
           continue;
 
         // Copy from last location to the current relocated location.
-        const Relocation &r = rels[i];
+        Relocation &r = rels[i];
         uint64_t size = r.offset - offset;
         memcpy(p, old.data() + offset, size);
         p += size;
-        offset = r.offset + remove;
+
+        int64_t skip = 0;
+        if (RelType newType = aux.relocTypes[i]) {
+          switch (newType) {
+          case R_LARCH_RELAX:
+            break;
+          case R_LARCH_PCREL20_S2:
+            skip = 4;
+            write32le(p, aux.writes[writesIdx++]);
+            // RelExpr is needed for relocating.
+            r.expr = r.sym->hasFlag(NEEDS_PLT) ? R_PLT_PC : R_PC;
+            break;
+          default:
+            llvm_unreachable("unsupported type");
+          }
+        }
+
+        p += skip;
+        offset = r.offset + skip + remove;
       }
       memcpy(p, old.data() + offset, old.size() - offset);