lld/AMDGPU: Fix asserts if no object files are involved in link

[arsenm]

Summary

Fixes issue 47690. The reproduction steps produced a shared object
from clang directly, and then fed the shared object back into
lld. With no regular object files, this assert was hit. I'm not sure
if we need to or should be looking for equivalent fields in shared
objects.

[MaskRay]

See D71101 for a similar change. Can you move the test to amdgpu-elf-flags*.s? Newer tests use ## for non-RUN non-CHECK comments to make them stand out (and potentially help future FileCheck feature to detect disabled RUN/CHECK lines due to typo)

[MaskRay]

lld/test/ELF/amdgpu-no-object-files-linked.s

10 ↗

(On Diff #421577)

Use llvm-readobj -h to test flags even if it is 0.

[arsenm]

Merge tests

[MaskRay]

LGTM with some nits

lld/test/ELF/amdgpu-elf-flags.s

5

Nit: for FileCheck options -- is more common

11

Align the values like:

# FIRSTLINK:      Flags [
# FIRSTLINK-NEXT:   EF_AMDGPU_MACH_AMDGCN_GFX803 (0x2A)
# FIRSTLINK-NEXT: ]

15

ditto

[arsenm]

lld/test/ELF/amdgpu-elf-flags.s

5

I don't think so. I'd guess across llvm about 10% use -- for check-prefix

[arsenm]

63fe6d7eae2991f6d1749a413a72c4f26a5a457f

[MaskRay]

lld/test/ELF/amdgpu-elf-flags.s

5

% rg 'FileCheck.* -check' -l | wc -l
7261
% rg 'FileCheck.* --check' -l | wc -l
12039

In lld the ratio is even larger. Not needed to change now.

lld/ELF/Arch/AMDGPU.cpp

if (retSramEcc == EF_AMDGPU_FEATURE_SRAMECC_UNSUPPORTED_V4 ||
         retSramEcc = getEFlags(f) & EF_AMDGPU_FEATURE_SRAMECC_V4;
     }
   }
 
   return retMach | retXnack | retSramEcc;
 }
 
 uint32_t AMDGPU::calcEFlags() const {
-  assert(!objectFiles.empty());
+  if (objectFiles.empty())
+    return 0;
 
   uint8_t abiVersion = cast<ObjFile<ELF64LE>>(objectFiles[0])->getObj()
       .getHeader().e_ident[EI_ABIVERSION];
   switch (abiVersion) {
   case ELFABIVERSION_AMDGPU_HSA_V2:
   case ELFABIVERSION_AMDGPU_HSA_V3:
     return calcEFlagsV3();
   case ELFABIVERSION_AMDGPU_HSA_V4:

lld/ELF/SyntheticSections.cpp

static uint8_t getAbiVersion() {
   // MIPS non-PIC executable gets ABI version 1.
   if (config->emachine == EM_MIPS) {
     if (!config->isPic && !config->relocatable &&
         (config->eflags & (EF_MIPS_PIC | EF_MIPS_CPIC)) == EF_MIPS_CPIC)
       return 1;
     return 0;
   }
 
-  if (config->emachine == EM_AMDGPU) {
+  if (config->emachine == EM_AMDGPU && !objectFiles.empty()) {
     uint8_t ver = objectFiles[0]->abiVersion;
     for (InputFile *file : makeArrayRef(objectFiles).slice(1))
       if (file->abiVersion != ver)
         error("incompatible ABI version: " + toString(file));
     return ver;
   }
 
   return 0;

lld/test/ELF/amdgpu-elf-flags.s

 # REQUIRES: amdgpu
 # RUN: llvm-mc -triple amdgcn-amd-amdhsa -mcpu=gfx803 --amdhsa-code-object-version=2 -filetype=obj %S/Inputs/amdgpu-kernel-0.s -o %t-0.o
 # RUN: llvm-mc -triple amdgcn-amd-amdhsa -mcpu=gfx803 --amdhsa-code-object-version=2 -filetype=obj %S/Inputs/amdgpu-kernel-1.s -o %t-1.o
 # RUN: ld.lld -shared %t-0.o %t-1.o -o %t.so
-# RUN: llvm-readobj --file-headers %t.so | FileCheck %s
+# RUN: llvm-readobj --file-headers %t.so | FileCheck -check-prefix=FIRSTLINK %s
 
-# CHECK: Flags [
-# CHECK:   EF_AMDGPU_MACH_AMDGCN_GFX803 (0x2A)
-# CHECK: ]
+## Try to link again where there are no object file inputs, only a shared library. Issue 47690
+# RUN: ld.lld -shared %t.so -o - | llvm-readobj -h - | FileCheck -check-prefix=SECONDLINK %s
+
+
+# FIRSTLINK: Flags [
+# FIRSTLINK-NEXT:   EF_AMDGPU_MACH_AMDGCN_GFX803 (0x2A)
+# FIRSTLINK-NEXT: ]
+
+# SECONDLINK: Flags [ (0x0)
+# SECONDLINK-NEXT: ]