From 8dcea93252a9ea7dff57e85220a719e2a5e8ab41 Mon Sep 17 00:00:00 2001 From: "H.J. Lu" Date: Fri, 15 May 2015 03:17:31 -0700 Subject: Add -mshared option to x86 ELF assembler This patch adds -mshared option to x86 ELF assembler. By default, assembler will optimize out non-PLT relocations against defined non-weak global branch targets with default visibility. The -mshared option tells the assembler to generate code which may go into a shared library where all non-weak global branch targets with default visibility can be preempted. The resulting code is slightly bigger. This option only affects the handling of branch instructions. This Linux kernel patch is needed to create a working x86 Linux kernel if it hasn't been applied: diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S index ae6588b..b91a00c 100644 --- a/arch/x86/kernel/head_64.S +++ b/arch/x86/kernel/head_64.S @@ -339,8 +339,8 @@ early_idt_handlers: i = i + 1 .endr -/* This is global to keep gas from relaxing the jumps */ -ENTRY(early_idt_handler) +/* This is weak to keep gas from relaxing the jumps */ +WEAK(early_idt_handler) cld cmpl $2,(%rsp) # X86_TRAP_NMI -- gas/ * config/tc-i386.c (shared): New. (OPTION_MSHARED): Likewise. (elf_symbol_resolved_in_segment_p): Add relocation argument. Check PLT relocations and shared. (md_estimate_size_before_relax): Pass fragP->fr_var to elf_symbol_resolved_in_segment_p. (md_longopts): Add -mshared. (md_show_usage): Likewise. (md_parse_option): Handle OPTION_MSHARED. * doc/c-i386.texi: Document -mshared. gas/testsuite/ * gas/i386/i386.exp: Don't run pcrel for ELF targets. Run pcrel-elf, relax-4 and x86-64-relax-3 for ELF targets. * gas/i386/pcrel-elf.d: New file. * gas/i386/relax-4.d: Likewise. * gas/i386/x86-64-relax-3.d: Likewise. * gas/i386/relax-3.d: Pass -mshared to assembler. Updated. * gas/i386/x86-64-relax-2.d: Likewise. * gas/i386/relax-3.s: Add test for PLT relocation. --- gas/doc/c-i386.texi | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'gas/doc') diff --git a/gas/doc/c-i386.texi b/gas/doc/c-i386.texi index 1645c8c..ea08c63 100644 --- a/gas/doc/c-i386.texi +++ b/gas/doc/c-i386.texi @@ -298,6 +298,17 @@ The @code{.att_syntax} and @code{.intel_syntax} directives will take precedent. This option forces the assembler to add BND prefix to all branches, even if such prefix was not explicitly specified in the source code. +@cindex @samp{-mshared} option, i386 +@cindex @samp{-mshared} option, x86-64 +@item -mno-shared +On ELF target, the assembler normally optimizes out non-PLT relocations +against defined non-weak global branch targets with default visibility. +The @samp{-mshared} option tells the assembler to generate code which +may go into a shared library where all non-weak global branch targets +with default visibility can be preempted. The resulting code is +slightly bigger. This option only affects the handling of branch +instructions. + @cindex @samp{-mbig-obj} option, x86-64 @item -mbig-obj On x86-64 PE/COFF target this option forces the use of big object file -- cgit v1.1