Relax call36/tail36.

Instructions with relocation `R_LARCH_CALL36` may be relax as follows:
```
From:
  pcaddu18i $dest, %call36(foo)
    R_LARCH_CALL36, R_LARCH_RELAX
  jirl $r, $dest, 0
To:
  b/bl foo  # bl if r=$ra, b if r=$zero
    R_LARCH_B26
```

Author: ylzsx

lld/ELF/Arch/LoongArch.cpp

@@ -58,6 +58,8 @@ enum Op {
   LD_W = 0x28800000,
   LD_D = 0x28c00000,
   JIRL = 0x4c000000,
+  B = 0x50000000,
+  BL = 0x54000000,
 };
 
 enum Reg {
@@ -830,6 +832,37 @@ static void relaxPCHi20Lo12(Ctx &ctx, const InputSection &sec, size_t i,
   remove = 4;
 }
 
+// Relax code sequence.
+// From:
+//   pcaddu18i $ra, %call36(foo)
+//   jirl $ra, $ra, 0
+// To:
+//   b/bl foo
+static void relaxCall36(Ctx &ctx, const InputSection &sec, size_t i,
+                        uint64_t loc, Relocation &r, uint32_t &remove) {
+  const uint64_t symLocal =
+      (r.expr == R_PLT_PC ? r.sym->getPltVA(ctx) : r.sym->getVA(ctx)) +
+      r.addend;
+
+  const int64_t distance = symLocal - loc;
+  // Check if the distance aligns 4 bytes or exceeds the range of b[l].
+  if ((distance & 0x3) != 0 || !isInt<28>(distance))
+    return;
+
+  const uint32_t nextInsn = read32le(sec.content().data() + r.offset + 4);
+  if (getD5(nextInsn) == R_RA) {
+    // convert jirl to bl
+    sec.relaxAux->relocTypes[i] = R_LARCH_B26;
+    sec.relaxAux->writes.push_back(insn(BL, 0, 0, 0));
+    remove = 4;
+  } else if (getD5(nextInsn) == R_ZERO) {
+    // convert jirl to b
+    sec.relaxAux->relocTypes[i] = R_LARCH_B26;
+    sec.relaxAux->writes.push_back(insn(B, 0, 0, 0));
+    remove = 4;
+  }
+}
+
 static bool relax(Ctx &ctx, InputSection &sec) {
   const uint64_t secAddr = sec.getVA();
   const MutableArrayRef<Relocation> relocs = sec.relocs();
@@ -874,6 +907,10 @@ static bool relax(Ctx &ctx, InputSection &sec) {
       if (isPairRelaxable(relocs, i))
         relaxPCHi20Lo12(ctx, sec, i, loc, r, relocs[i + 2], remove);
       break;
+    case R_LARCH_CALL36:
+      if (relaxable(relocs, i))
+        relaxCall36(ctx, sec, i, loc, r, remove);
+      break;
     }
 
     // For all anchors whose offsets are <= r.offset, they are preceded by
@@ -977,6 +1014,10 @@ void LoongArch::finalizeRelax(int passes) const {
             // RelExpr is needed for relocating.
             r.expr = r.sym->hasFlag(NEEDS_PLT) ? R_PLT_PC : R_PC;
             break;
+          case R_LARCH_B26:
+            skip = 4;
+            write32le(p, aux.writes[writesIdx++]);
+            break;
           default:
             llvm_unreachable("unsupported type");
           }