[lld] [X86][LLD] Handle R_X86_64_CODE_4_GOTTPOFF relocation type (PR #116634)

[Shengchen Kan via llvm-commits]

================
@@ -547,44 +548,66 @@ void X86_64::relaxTlsGdToIe(uint8_t *loc, const Relocation &rel,
   }
 }
 
-// In some conditions, R_X86_64_GOTTPOFF relocation can be optimized to
-// R_X86_64_TPOFF32 so that it does not use GOT.
+// In some conditions, R_X86_64_GOTTPOFF/R_X86_64_CODE_4_GOTTPOFF relocation can
+// be optimized to R_X86_64_TPOFF32 so that it does not use GOT.
 void X86_64::relaxTlsIeToLe(uint8_t *loc, const Relocation &rel,
                             uint64_t val) const {
   uint8_t *inst = loc - 3;
   uint8_t reg = loc[-1] >> 3;
   uint8_t *regSlot = loc - 1;
 
-  // Note that ADD with RSP or R12 is converted to ADD instead of LEA
-  // because LEA with these registers needs 4 bytes to encode and thus
-  // wouldn't fit the space.
-
-  if (memcmp(inst, "\x48\x03\x25", 3) == 0) {
-    // "addq foo at gottpoff(%rip),%rsp" -> "addq $foo,%rsp"
-    memcpy(inst, "\x48\x81\xc4", 3);
-  } else if (memcmp(inst, "\x4c\x03\x25", 3) == 0) {
-    // "addq foo at gottpoff(%rip),%r12" -> "addq $foo,%r12"
-    memcpy(inst, "\x49\x81\xc4", 3);
-  } else if (memcmp(inst, "\x4c\x03", 2) == 0) {
-    // "addq foo at gottpoff(%rip),%r[8-15]" -> "leaq foo(%r[8-15]),%r[8-15]"
-    memcpy(inst, "\x4d\x8d", 2);
-    *regSlot = 0x80 | (reg << 3) | reg;
-  } else if (memcmp(inst, "\x48\x03", 2) == 0) {
-    // "addq foo at gottpoff(%rip),%reg -> "leaq foo(%reg),%reg"
-    memcpy(inst, "\x48\x8d", 2);
-    *regSlot = 0x80 | (reg << 3) | reg;
-  } else if (memcmp(inst, "\x4c\x8b", 2) == 0) {
-    // "movq foo at gottpoff(%rip),%r[8-15]" -> "movq $foo,%r[8-15]"
-    memcpy(inst, "\x49\xc7", 2);
-    *regSlot = 0xc0 | reg;
-  } else if (memcmp(inst, "\x48\x8b", 2) == 0) {
-    // "movq foo at gottpoff(%rip),%reg" -> "movq $foo,%reg"
-    memcpy(inst, "\x48\xc7", 2);
-    *regSlot = 0xc0 | reg;
+  if (rel.type == R_X86_64_GOTTPOFF) {
+    // Note that ADD with RSP or R12 is converted to ADD instead of LEA
+    // because LEA with these registers needs 4 bytes to encode and thus
+    // wouldn't fit the space.
+
+    if (memcmp(inst, "\x48\x03\x25", 3) == 0) {
+      // "addq foo at gottpoff(%rip),%rsp" -> "addq $foo,%rsp"
+      memcpy(inst, "\x48\x81\xc4", 3);
+    } else if (memcmp(inst, "\x4c\x03\x25", 3) == 0) {
+      // "addq foo at gottpoff(%rip),%r12" -> "addq $foo,%r12"
+      memcpy(inst, "\x49\x81\xc4", 3);
+    } else if (memcmp(inst, "\x4c\x03", 2) == 0) {
+      // "addq foo at gottpoff(%rip),%r[8-15]" -> "leaq foo(%r[8-15]),%r[8-15]"
+      memcpy(inst, "\x4d\x8d", 2);
+      *regSlot = 0x80 | (reg << 3) | reg;
+    } else if (memcmp(inst, "\x48\x03", 2) == 0) {
+      // "addq foo at gottpoff(%rip),%reg -> "leaq foo(%reg),%reg"
+      memcpy(inst, "\x48\x8d", 2);
+      *regSlot = 0x80 | (reg << 3) | reg;
+    } else if (memcmp(inst, "\x4c\x8b", 2) == 0) {
+      // "movq foo at gottpoff(%rip),%r[8-15]" -> "movq $foo,%r[8-15]"
+      memcpy(inst, "\x49\xc7", 2);
+      *regSlot = 0xc0 | reg;
+    } else if (memcmp(inst, "\x48\x8b", 2) == 0) {
+      // "movq foo at gottpoff(%rip),%reg" -> "movq $foo,%reg"
+      memcpy(inst, "\x48\xc7", 2);
+      *regSlot = 0xc0 | reg;
+    } else {
+      Err(ctx)
+          << getErrorLoc(ctx, loc - 3)
+          << "R_X86_64_GOTTPOFF must be used in MOVQ or ADDQ instructions only";
+    }
   } else {
----------------
KanRobert wrote:

`relaxTlsIeToLe` is only called for `R_X86_64_CODE_4_GOTTPOFF` and `R_X86_64_GOTTPOFF`.  Probably, the code can be more readable by removing the first `assert`  below and use
```
else if (rel.type == R_X86_64_CODE_4_GOTTPOFF) {
} else {
  llvm_unreachable("Unsupported relocation type!")
 }
 
 And I think the following R_X86_64_CODE_6_GOTTPOFF support can benefit from this too. 
 
 The second `assert` should not be there, the diagnostic info should be always be reported by code like line 606
```
        Err(ctx) << getErrorLoc(ctx, loc - 3)
                 << "R_X86_64_CODE_4_GOTTPOFF must be used in MOVQ or ADDQ "
                    "instructions only";
``` 
, otherwise the error would be hidden in the release build.  BTW,  it seems `loc - 4` should be used here. 


https://github.com/llvm/llvm-project/pull/116634