[JITLink][LoongArch] Ignore R_LARCH_RELAX and check R_LARCH_ALIGN

[zhaoqi5]

Linker relaxation is not implemented for jitlink now (maybe implement in the future). But if relaxation is enabled by clang, R_LARCH_RELAX and R_LARCH_ALIGN relocations will be emitted. So we just ignore linker relaxation and check the alignment now.

If not, interpreting C++ code using clang-repl when relaxation is enabled will occur error: JIT session error: Unsupported loongarch relocation:102: R_LARCH_ALIGN.

Similar to: f5b5398.

[llvmbot]

@llvm/pr-subscribers-backend-loongarch

Author: ZhaoQi (zhaoqi5)

Changes

Linker relaxation is not implemented for jitlink now (maybe implement in the future). But if relaxation is enabled by clang, R_LARCH_RELAX and R_LARCH_ALIGN relocations will be emitted. So we just ignore linker relaxation and check the alignment now.

If not, interpreting C++ code using clang-repl when relaxation is enabled will occur error: JIT session error: Unsupported loongarch relocation:102: R_LARCH_ALIGN.

Similar to: f5b5398.

---

Full diff: https://github.com/llvm/llvm-project/pull/121097.diff

1 Files Affected:

• (modified) llvm/lib/ExecutionEngine/JITLink/ELF_loongarch.cpp (+24-6)

diff --git a/llvm/lib/ExecutionEngine/JITLink/ELF_loongarch.cpp b/llvm/lib/ExecutionEngine/JITLink/ELF_loongarch.cpp
index 56c32aeecf55a8..1f4e34f41ab2b5 100644
--- a/llvm/lib/ExecutionEngine/JITLink/ELF_loongarch.cpp
+++ b/llvm/lib/ExecutionEngine/JITLink/ELF_loongarch.cpp
@@ -95,6 +95,30 @@ class ELFLinkGraphBuilder_loongarch : public ELFLinkGraphBuilder<ELFT> {
                             Block &BlockToFix) {
     using Base = ELFLinkGraphBuilder<ELFT>;
 
+    uint32_t Type = Rel.getType(false);
+    // We do not implement linker relaxation for jitlink now, except what is
+    // required for alignment (see below).
+    if (Type == ELF::R_LARCH_RELAX)
+      return Error::success();
+
+    int64_t Addend = Rel.r_addend;
+    if (Type == ELF::R_LARCH_ALIGN) {
+      uint64_t Alignment = PowerOf2Ceil(Addend);
+      // FIXME: Implement support for ensuring alignment together with linker
+      // relaxation. Addend is always 28 in the most common case when
+      // interpreting C++ code in clang-repl.
+      if (Alignment > 32)
+        return make_error<JITLinkError>(
+            formatv("Unsupported relocation R_LARCH_ALIGN with alignment {0} "
+                    "larger than 32 (addend: {1})",
+                    Alignment, Addend));
+      return Error::success();
+    }
+
+    Expected<loongarch::EdgeKind_loongarch> Kind = getRelocationKind(Type);
+    if (!Kind)
+      return Kind.takeError();
+
     uint32_t SymbolIndex = Rel.getSymbol(false);
     auto ObjSymbol = Base::Obj.getRelocationSymbol(Rel, Base::SymTabSec);
     if (!ObjSymbol)
@@ -109,12 +133,6 @@ class ELFLinkGraphBuilder_loongarch : public ELFLinkGraphBuilder<ELFT> {
                   Base::GraphSymbols.size()),
           inconvertibleErrorCode());
 
-    uint32_t Type = Rel.getType(false);
-    Expected<loongarch::EdgeKind_loongarch> Kind = getRelocationKind(Type);
-    if (!Kind)
-      return Kind.takeError();
-
-    int64_t Addend = Rel.r_addend;
     auto FixupAddress = orc::ExecutorAddr(FixupSect.sh_addr) + Rel.r_offset;
     Edge::OffsetT Offset = FixupAddress - BlockToFix.getAddress();
     Edge GE(*Kind, Offset, *GraphSymbol, Addend);

[zhaoqi5]

R_LARCH_ALIGN cannot be simply ignored. Although tests of clang-repl do not meet with error.
But error may occur. For example:

    .text
    .globl main
    .type main,@function
main:
    nop 
    ret 

    .globl br16
br16:

test_br16:
    .p2align 4
    la.pcrel $a0, br16
    .skip (1 << 17 - 16) 
    beq $t1, $t2, %b16(br16)

• Without relax: llvm-mc --triple=loongarch64 --filetype=obj t.s -o - | llvm-objdump -d -, two nops should be inserted.

0000000000000000 <main>:
       0: 00 00 40 03  	nop
       4: 20 00 00 4c  	ret

0000000000000008 <test_br16>:
       8: 00 00 40 03  	nop
       c: 00 00 40 03  	nop
      10: 04 00 00 1a  	pcalau12i	$a0, 0
      14: 84 00 c0 02  	addi.d	$a0, $a0, 0
		...
   20008: ae 01 00 58  	beq	$t1, $t2, 0 <test_br16+0x20000>

• But with relax: llvm-mc --triple=loongarch64 --filetype=obj -mattr=+relax t.s -o - | llvm-objdump -d -, three nops are inserted. The linker is expected to remove one nop to align pcalau12i.

0000000000000000 <main>:
       0: 00 00 40 03  	nop
       4: 20 00 00 4c  	ret

0000000000000008 <test_br16>:
       8: 00 00 40 03  	nop
       c: 00 00 40 03  	nop
      10: 00 00 40 03  	nop
      14: 04 00 00 1a  	pcalau12i	$a0, 0
      18: 84 00 c0 02  	addi.d	$a0, $a0, 0
		...
   2000c: ae 01 00 58  	beq	$t1, $t2, 0 <test_br16+0x20004>

If we ignore R_LARCH_ALIGN in jitlink. Nops will not be removed and beq will be out of range while running llvm-jitlink t.o.

llvm-jitlink error: In graph t.o, section .text: relocation target "br16" at address 0x7fffa4000008 is out of range of Branch16PCRel fixup at 0x7fffa402000c (main, 0x7fffa4000000 + 0x2000c)