[AMDGPU][MC] Generate relative relocations for allocatable (more particularly, eh_frame) sections

[arsenm]

Can you also add a codegen test that hits this?

[jmmartinez]

In D142453#4086520, @arsenm wrote:

Can you also add a codegen test that hits this?

Unfortunately, there is no codegen path for this (for the moment).

This patch is related to the work I started in https://reviews.llvm.org/D139024, which forces the emission of CFI entries in the .eh_frame section for functions with the uwtable attribute.

The problem is that without this patch https://reviews.llvm.org/D139024 breaks the compilation of AMDGPU programs when the sanitizers are enabled (lld raises an error due to the problematic relocations).

With the work from https://reviews.llvm.org/D139024 , this case will be triggered for functions with the uwtable attribute.

[scott.linder]

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUELFObjectWriter.cpp

73

I'm not certain I understand why there are some fixup kinds which encode their PCRelative-ness (e.g. FK_PCRel_4) and some which don't (e.g. FK_Data_4), and yet there is also a separate channel where a fixup that is not intrinsically PCRelative can be tagged as FixupKindFlags::FKF_IsPCRel (here the value of IsPCRel).

The representation seems to be decades old, so I will just take it on faith that it makes sense, and in light of it the change LGTM!

llvm/lib/Target/AMDGPU/MCTargetDesc/AMDGPUELFObjectWriter.cpp

case MCSymbolRefExpr::VK_AMDGPU_REL64:
     return ELF::R_AMDGPU_REL64;
   }
 
   MCFixupKind Kind = Fixup.getKind();
   if (Kind >= FirstLiteralRelocationKind)
     return Kind - FirstLiteralRelocationKind;
   switch (Kind) {
   default: break;
   case FK_PCRel_4:
     return ELF::R_AMDGPU_REL32;
   case FK_Data_4:
   case FK_SecRel_4:
-    return ELF::R_AMDGPU_ABS32;
+    return IsPCRel ? ELF::R_AMDGPU_REL32 : ELF::R_AMDGPU_ABS32;
   case FK_Data_8:
-    return ELF::R_AMDGPU_ABS64;
+    return IsPCRel ? ELF::R_AMDGPU_REL64 : ELF::R_AMDGPU_ABS64;
   }
 
   if (Fixup.getTargetKind() == AMDGPU::fixup_si_sopp_br) {
     const auto *SymA = Target.getSymA();
     assert(SymA);
 
     if (SymA->getSymbol().isUndefined()) {
       Ctx.reportError(Fixup.getLoc(), Twine("undefined label '") +

llvm/test/MC/AMDGPU/cfi_reloc.s

+// RUN: llvm-mc -filetype=obj -triple amdgcn-- -mcpu=kaveri -show-encoding %s | llvm-readobj -r - | FileCheck %s
+
+// CHECK: Relocations [
+// CHECK: .rel.eh_frame {
+// CHECK: R_AMDGPU_REL32 .text
+// CHECK: }
+// CHECK: .rel.debug_frame {
+// CHECK: R_AMDGPU_ABS64 .text
+// CHECK: }
+// CHECK: ]
+
+kernel:
+  .cfi_startproc
+  .cfi_sections .debug_frame, .eh_frame
+  s_endpgm
+  .cfi_endproc
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