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[X86] -fno-plt: use GOT __tls_get_addr only if GOTPCRELX is enabled

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Summary:
As of binutils 2.32, ld has a bogus TLS relaxation error when the GD/LD
code sequence using R_X86_64_GOTPCREL (instead of R_X86_64_GOTPCRELX) is
attempted to be relaxed to IE/LE (binutils PR24784). gold and lld are good.

In gcc/config/i386/i386.md, there is a configure-time check of as/ld
support and the GOT relaxation will not be used if as/ld doesn't support
it:

    if (flag_plt || !HAVE_AS_IX86_TLS_GET_ADDR_GOT)
      return "call\t%P2";
    return "call\t{*%p2@GOT(%1)|[DWORD PTR %p2@GOT[%1]]}";

In clang, -DENABLE_X86_RELAX_RELOCATIONS=OFF is the default. The ld.bfd
bogus error can be reproduced with:

    thread_local int a;
    int main() { return a; }

clang -fno-plt -fpic a.cc -fuse-ld=bfd

GOTPCRELX gained relative good support in 2016, which is considered
relatively new.  It is even difficult to conditionally default to
-DENABLE_X86_RELAX_RELOCATIONS=ON due to cross compilation reasons. So
work around the ld.bfd bug by only using GOT when GOTPCRELX is enabled.

Reviewers: dalias, hjl.tools, nikic, rnk

Reviewed By: nikic

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64304

llvm-svn: 365752
This commit is contained in:
Fangrui Song 2019-07-11 10:10:09 +00:00
parent 4328722a26
commit a6a4c14893
2 changed files with 19 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
lib/Target/X86/X86MCInstLower.cpp
View File

@ -705,7 +705,14 @@ void X86AsmPrinter::LowerTlsAddr(X86MCInstLower &MCInstLowering,
const MCSymbolRefExpr *Sym = MCSymbolRefExpr::create(
MCInstLowering.GetSymbolFromOperand(MI.getOperand(3)), SRVK, Ctx);
bool UseGot = MMI->getModule()->getRtLibUseGOT();
// As of binutils 2.32, ld has a bogus TLS relaxation error when the GD/LD
// code sequence using R_X86_64_GOTPCREL (instead of R_X86_64_GOTPCRELX) is
// attempted to be relaxed to IE/LE (binutils PR24784). Work around the bug by
// only using GOT when GOTPCRELX is enabled.
// TODO Delete the workaround when GOTPCRELX becomes commonplace.
bool UseGot = MMI->getModule()->getRtLibUseGOT() &&
Ctx.getAsmInfo()->canRelaxRelocations();
if (Is64Bits) {
bool NeedsPadding = SRVK == MCSymbolRefExpr::VK_TLSGD;

13
test/CodeGen/X86/tls-no-plt.ll
View File

@ -1,5 +1,10 @@
; RUN: llc < %s -mtriple=i386-linux-musl -relocation-model=pic | FileCheck --check-prefixes=CHECK,X86 %s
; RUN: llc < %s -mtriple=x86_64-linux-musl -relocation-model=pic | FileCheck --check-prefixes=CHECK,X64 %s
; RUN: llc < %s -mtriple=i386-linux-musl -relocation-model=pic -relax-elf-relocations=true | FileCheck --check-prefixes=CHECK,X86 %s
; RUN: llc < %s -mtriple=x86_64-linux-musl -relocation-model=pic -relax-elf-relocations=true | FileCheck --check-prefixes=CHECK,X64 %s
;; If GOTPCRELX is disabled, don't use GOT for __tls_get_addr to work around
;; a ld.bfd bug (binutils PR24784).
; RUN: llc < %s -mtriple=i386-linux-musl -relocation-model=pic | FileCheck --check-prefixes=CHECK,X86-PLT %s
; RUN: llc < %s -mtriple=x86_64-linux-musl -relocation-model=pic | FileCheck --check-prefixes=CHECK,X64-PLT %s
@gd = thread_local global i32 0
@ld = internal thread_local global i32 0
@ -9,9 +14,11 @@ entry:
; CHECK-LABEL: get_gd:
; X86: leal gd@TLSGD(%ebx), %eax
; X86: calll *___tls_get_addr@GOT(%ebx)
; X86-PLT: calll ___tls_get_addr@PLT
; X64: leaq gd@TLSGD(%rip), %rdi
; X64: callq *__tls_get_addr@GOTPCREL(%rip)
; X64-PLT: callq __tls_get_addr@PLT
ret i32* @gd
}
@ -20,9 +27,11 @@ define i32* @get_ld() {
; CHECK-LABEL: get_ld:
; X86: leal ld@TLSLDM(%ebx), %eax
; X86: calll *___tls_get_addr@GOT(%ebx)
; X86-PLT: calll ___tls_get_addr@PLT
; X64: leaq ld@TLSLD(%rip), %rdi
; X64: callq *__tls_get_addr@GOTPCREL(%rip)
; X64-PLT: callq __tls_get_addr@PLT
ret i32* @ld
}