[MC][ELF] Mark GNU ABI if ifunc are used

[zatrazz]

Summary

Similar to D107861. Some tools required the GNU ABI mark to output
the symbol is a IFUNC type correctly (for instance binutils readelf).

[MaskRay]

The SHF_GNU_RETAIN change is more so that GNU ld recognizes its semantics, less about the readelf -s output. With many systems adopting GNU ABI features, I do not know whether it makes significant sense to "upgrade" ELFOSABI_NONE to ELFOSABI_GNU.

[zatrazz]

In D131162#3708701, @MaskRay wrote:

The SHF_GNU_RETAIN change is more so that GNU ld recognizes its semantics, less about the readelf -s output. With many systems adopting GNU ABI features, I do not know whether it makes significant sense to "upgrade" ELFOSABI_NONE to ELFOSABI_GNU.

I came up with this change mainly to make glibc configure script to detect that clang can actually use the ifunc attribute, otherwise it will use the old inline asm macros which might be subpar regarding debug information [1].

And I think this change also aligns with the recent discussion gABI where iFUNC does make the object ELFOSABI_GNU.

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=configure.ac;h=eb5bc6a1313acb3f3614645739adeb7638f19617;hb=HEAD#l657

[emaste]

IMO we ought to test that FreeBSD (or some other non-OS/ABI NONE case) isn't changed to GNU, similar to the test in D97976.

llvm/lib/MC/MCELFStreamer.cpp

case MCSA_Local:
     break;
 
   case MCSA_ELF_TypeFunction:
     Symbol->setType(CombineSymbolTypes(Symbol->getType(), ELF::STT_FUNC));
     break;
 
   case MCSA_ELF_TypeIndFunction:
     Symbol->setType(CombineSymbolTypes(Symbol->getType(), ELF::STT_GNU_IFUNC));
+    getAssembler().getWriter().markGnuAbi();
     break;
 
   case MCSA_ELF_TypeObject:
     Symbol->setType(CombineSymbolTypes(Symbol->getType(), ELF::STT_OBJECT));
     break;
 
   case MCSA_ELF_TypeTLS:
     Symbol->setType(CombineSymbolTypes(Symbol->getType(), ELF::STT_TLS));

llvm/test/MC/ELF/ifunc-gnu.s

+# REQUIRES: x86-registered-target
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux-gnu %s -o -| llvm-readelf -hs - | FileCheck %s
+
+.text
+
+.type  foo_impl,@function
+foo_impl:
+  ret
+
+.type  foo_resolver,@function
+foo_resolver:
+  mov $foo_impl, %rax
+  ret
+
+.type  foo,@gnu_indirect_function
+.set   foo,foo_resolver
+
+## ELFOSABI_NONE is changed to ELFOSABI_GNU. Other OSABI values are unchanged.
+# CHECK:      OS/ABI: UNIX - GNU
+# CHECK:      FUNC    LOCAL  DEFAULT    2 foo_impl
+# CHECK-NEXT: FUNC    LOCAL  DEFAULT    2 foo_resolver
+# CHECK-NEXT: IFUNC   LOCAL  DEFAULT    2 foo