[TargetLoweringObjectFileImpl] Make .llvmbc and .llvmcmd non-SHF_ALLOC

[MaskRay]

Summary

There are two ways .llvmbc can be produced:

• clang -c -fembed-bitcode=all (which also produces .llvmcmd)
• LTO backend: ld.lld -mllvm -lto-embed-bitcode or -plugin-opt=-lto-embed-bitcode

.llvmbc and .llvmcmd have the SHF_ALLOC flag, so they can be dropped by
--gc-sections.

This patch sets SectionKind::Metadata to drop the SHF_ALLOC flag. This
is conceptually correct: the two sections are not part of the process
image, so SHF_ALLOC is not appropriate.

test/LTO/X86/embed-bitcode.ll: changed llvm-objcopy -O binary --only-section to
llvm-objcopy --dump-section. -O binary does not dump non-SHF_ALLOC sections.

[tejohnson]

lgtm

[MaskRay]

Fix embed-bitcode.ll (use --dump-section instead) and add a note

[nikic]

Could you please take a look at https://bugs.llvm.org/show_bug.cgi?id=51207, which has been caused by this change?

[MaskRay]

In D86374#2913034, @nikic wrote:

Could you please take a look at https://bugs.llvm.org/show_bug.cgi?id=51207, which has been caused by this change?

Not very related to this patch.

The Rust code abuses a previous LLVM behavior which was changed by D100944 (similar to D72194).
I will make a comment to https://bugs.llvm.org/show_bug.cgi?id=51207

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

static SectionKind getELFKindForNamedSection(StringRef Name, SectionKind K) {
   // both gas and MC will produce a section with no flags. Given
   // section(".eh_frame") gcc will produce:
   //
   //   .section   .eh_frame,"a",@progbits
 
   if (Name == getInstrProfSectionName(IPSK_covmap, Triple::ELF,
                                       /*AddSegmentInfo=*/false) ||
       Name == getInstrProfSectionName(IPSK_covfun, Triple::ELF,
-                                      /*AddSegmentInfo=*/false))
+                                      /*AddSegmentInfo=*/false) ||
+      Name == ".llvmbc" || Name == ".llvmcmd")
     return SectionKind::getMetadata();
 
   if (Name.empty() || Name[0] != '.') return K;
 
   // Default implementation based on some magic section names.
   if (Name == ".bss" ||
       Name.startswith(".bss.") ||
       Name.startswith(".gnu.linkonce.b.") ||

llvm/test/CodeGen/X86/embed-bitcode.ll

+; RUN: llc -filetype=obj -mtriple=x86_64 %s -o %t
+; RUN: llvm-readelf -S %t | FileCheck %s
+
+; CHECK:      .text    PROGBITS 0000000000000000 [[#%x,OFF:]] 000000 00 AX 0
+; CHECK-NEXT: .llvmbc  PROGBITS 0000000000000000 [[#%x,OFF:]] 000004 00    0
+; CHECK-NEXT: .llvmcmd PROGBITS 0000000000000000 [[#%x,OFF:]] 000005 00    0
+
[e-mail] = private constant [4 x i8] c"BC\C0\DE", section ".llvmbc"
[e-mail] = private constant [5 x i8] c"-cc1\00", section ".llvmcmd"
[e-mail] = appending global [2 x i8*] [i8* getelementptr inbounds ([4 x i8], [4 x i8]* @llvm.embedded.module, i32 0, i32 0), i8* getelementptr inbounds ([5 x i8], [5 x i8]* @llvm.cmdline, i32 0, i32 0)], section "llvm.metadata"

llvm/test/LTO/X86/embed-bitcode.ll

 ; RUN: llvm-as %s -o %t1.o
 ; RUN: llvm-as %p/Inputs/start-lib1.ll -o %t2.o
 ; RUN: llvm-as %p/Inputs/start-lib2.ll -o %t3.o
 
 ; RUN: llvm-lto2 run -r %t1.o,_start,px -r %t2.o,foo,px -r %t3.o,bar,px -r %t2.o,bar,lx -o %t3 %t1.o %t2.o %t3.o
 ; RUN: llvm-readelf -S %t3.0 | FileCheck %s --implicit-check-not=.llvmbc
 
 ; RUN: llvm-lto2 run -r %t1.o,_start,px -r %t2.o,foo,px -r %t3.o,bar,px -r %t2.o,bar,lx -lto-embed-bitcode=false -o %t3 %t1.o %t2.o %t3.o
 ; RUN: llvm-readelf -S %t3.0 | FileCheck %s --implicit-check-not=.llvmbc
 
 ; RUN: llvm-lto2 run -r %t1.o,_start,px -r %t2.o,foo,px -r %t3.o,bar,px -r %t2.o,bar,lx -lto-embed-bitcode -o %t3 %t1.o %t2.o %t3.o
 ; RUN: llvm-readelf -S %t3.0 | FileCheck %s --check-prefix=CHECK-ELF
-; RUN: llvm-objcopy -O binary -j .llvmbc %t3.0 %t-embedded.bc
+; RUN: llvm-objcopy --dump-section=.llvmbc=%t-embedded.bc %t3.0 /dev/null
 ; RUN: llvm-dis %t-embedded.bc -o - | FileCheck %s --check-prefix=CHECK-LL
 
-; CHECK-ELF: .text
-; CHECK-ELF: .llvmbc
+; CHECK-ELF:      .text   PROGBITS 0000000000000000 [[#%x,OFF:]] [[#%x,SIZE:]] 00 AX 0
+; CHECK-ELF-NEXT: .llvmbc PROGBITS 0000000000000000 [[#%x,OFF:]] [[#%x,SIZE:]] 00    0
 
 ; CHECK-LL: @_start
 ; CHECK-LL: @foo
 ; CHECK-LL: @bar
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 define void @_start() {
   ret void
 }