[lld][RISCV] Print error when encountering R_RISCV_ALIGN

Summary:
Unlike R_RISCV_RELAX, which is a linker hint, R_RISCV_ALIGN requires the
support of the linker even when ignoring all R_RISCV_RELAX relocations.
This is because the compiler emits as many NOPs as may be required for
the requested alignment, more than may be required pre-relaxation, to
allow for the target becoming more unaligned after relaxing earlier
sequences. This means that the target is often not initially aligned in
the object files, and so the R_RISCV_ALIGN relocations cannot just be
ignored. Since we do not support linker relaxation, we must turn these
into errors.

Reviewers: ruiu, MaskRay, espindola

Reviewed By: MaskRay

Subscribers: grimar, Jim, emaste, arichardson, asb, rbar, johnrusso, simoncook, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, rkruppe, PkmX, jocewei, psnobl, benna, lenary, s.egerton, pzheng, sameer.abuasal, apazos, luismarques, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71820

lld/ELF/Arch/RISCV.cpp

@@ -236,9 +236,15 @@ RelExpr RISCV::getRelExpr(const RelType type, const Symbol &s,
   case R_RISCV_TPREL_LO12_S:
     return R_TLS;
   case R_RISCV_RELAX:
-  case R_RISCV_ALIGN:
   case R_RISCV_TPREL_ADD:
     return R_NONE;
+  case R_RISCV_ALIGN:
+    // Not just a hint; always padded to the worst-case number of NOPs, so may
+    // not currently be aligned, and without linker relaxation support we can't
+    // delete NOPs to realign.
+    errorOrWarn(getErrorLocation(loc) + "relocation R_RISCV_ALIGN requires "
+                "unimplemented linker relaxation; recompile with -mno-relax");
+    return R_NONE;
   default:
     error(getErrorLocation(loc) + "unknown relocation (" + Twine(type) +
           ") against symbol " + toString(s));
@@ -431,7 +437,6 @@ void RISCV::relocateOne(uint8_t *loc, const RelType type,
     write64le(loc, val - dtpOffset);
     break;
 
-  case R_RISCV_ALIGN:
   case R_RISCV_RELAX:
     return; // Ignored (for now)
 

lld/test/ELF/riscv-reloc-align.s

@@ -0,0 +1,12 @@
+# REQUIRES: riscv
+
+# RUN: llvm-mc -filetype=obj -triple=riscv32 -mattr=+relax %s -o %t.o
+# RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
+
+# CHECK: relocation R_RISCV_ALIGN requires unimplemented linker relaxation
+
+.global _start
+_start:
+    nop
+    .balign 8
+    nop