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Re: PING: [PATCH] x86: Force __x86_indirect_thunk_reg for function call via GOT
- From: Jan Hubicka <hubicka at ucw dot cz>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>
- Cc: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Tue, 20 Mar 2018 15:48:39 +0100
- Subject: Re: PING: [PATCH] x86: Force __x86_indirect_thunk_reg for function call via GOT
- References: <CAMe9rOrCMWn_aFJNNsozpuO4tQD-2afKg98GNTCpFz+3o3Cmxg@mail.gmail.com>
> On Tue, Feb 27, 2018 at 11:39 AM, H.J. Lu <hongjiu.lu@intel.com> wrote:
> > For x86 targets, when -fno-plt is used, external functions are called
> > via GOT slot, in 64-bit mode:
> >
> > [bnd] call/jmp *foo@GOTPCREL(%rip)
> >
> > and in 32-bit mode:
> >
> > [bnd] call/jmp *foo@GOT[(%reg)]
> >
> > With -mindirect-branch=, they are converted to, in 64-bit mode:
> >
> > pushq foo@GOTPCREL(%rip)
> > [bnd] jmp __x86_indirect_thunk[_bnd]
> >
> > and in 32-bit mode:
> >
> > pushl foo@GOT[(%reg)]
> > [bnd] jmp __x86_indirect_thunk[_bnd]
> >
> > which were incompatible with CFI. In 64-bit mode, since R11 is a scratch
> > register, we generate:
> >
> > movq foo@GOTPCREL(%rip), %r11
> > [bnd] call/jmp __x86_indirect_thunk_[bnd_]r11
> >
> > instead. We do it in ix86_output_indirect_branch so that we can use
> > the newly proposed R_X86_64_THUNK_GOTPCRELX relocation:
> >
> > https://groups.google.com/forum/#!topic/x86-64-abi/eED5lzn3_Mg
> >
> > movq foo@OTPCREL_THUNK(%rip), %r11
> > [bnd] call/jmp __x86_indirect_thunk_[bnd_]r11
> >
> > to load GOT slot into R11. If foo is defined locally, linker can can
> > convert
> >
> > movq foo@GOTPCREL_THUNK(%rip), %reg
> > call/jmp __x86_indirect_thunk_reg
> >
> > to
> >
> > call/jmp foo
> > nop 0L(%rax)
> >
> > In 32-bit mode, since all caller-saved registers, EAX, EDX and ECX, may
> > used to function parameters, there is no scratch register available. For
> > -fno-plt -fno-pic -mindirect-branch=, we expand external function call
> > to:
> >
> > movl foo@GOT, %reg
> > [bnd] call/jmp *%reg
> >
> > so that it can be converted to
> >
> > movl foo@GOT, %reg
> > [bnd] call/jmp __x86_indirect_thunk_[bnd_]reg
> >
> > in ix86_output_indirect_branch. Since this is performed during RTL
> > expansion, other instructions may be inserted between movl and call/jmp.
> > Linker optimization isn't always possible.
> >
> > Tested on i686 and x86-64. OK for trunk?
> >
> >
> > H.J.
> > ---
> > gcc/
> >
> > PR target/83970
> > * config/i386/constraints.md (Bs): Allow GOT_memory_operand
> > for TARGET_LP64 with indirect branch conversion.
> > (Bw): Likewise.
> > * config/i386/i386.c (ix86_expand_call): Handle -fno-plt with
> > -mindirect-branch=.
> > (ix86_nopic_noplt_attribute_p): Likewise.
> > (ix86_output_indirect_branch): In 64-bit mode, convert function
> > call via GOT with R11 as a scratch register using
> > __x86_indirect_thunk_r11.
> > (ix86_output_call_insn): In 64-bit mode, set xasm to NULL when
> > calling ix86_output_indirect_branch with function call via GOT.
> > * config/i386/i386.md (*call_got_thunk): New call pattern for
> > TARGET_LP64 with indirect branch conversion.
> > (*call_value_got_thunk): Likewise.
> >
> > gcc/testsuite/
> >
> > PR target/83970
> > * gcc.target/i386/indirect-thunk-5.c: Updated.
> > * gcc.target/i386/indirect-thunk-6.c: Likewise.
> > * gcc.target/i386/indirect-thunk-bnd-3.c: Likewise.
> > * gcc.target/i386/indirect-thunk-bnd-4.c: Likewise.
> > * gcc.target/i386/indirect-thunk-extern-5.c: Likewise.
> > * gcc.target/i386/indirect-thunk-extern-6.c: Likewise.
> > * gcc.target/i386/indirect-thunk-inline-5.c: Likewise.
> > * gcc.target/i386/indirect-thunk-inline-6.c: Likewise.
> > * gcc.target/i386/indirect-thunk-13.c: New test.
> > * gcc.target/i386/indirect-thunk-14.c: Likewise.
> > * gcc.target/i386/indirect-thunk-bnd-5.c: Likewise.
> > * gcc.target/i386/indirect-thunk-bnd-6.c: Likewise.
> > * gcc.target/i386/indirect-thunk-extern-11.c: Likewise.
> > * gcc.target/i386/indirect-thunk-extern-12.c: Likewise.
> > * gcc.target/i386/indirect-thunk-inline-8.c: Likewise.
> > * gcc.target/i386/indirect-thunk-inline-9.c: Likewise.
Patch is OK. I am just bit worried how many additional features we will need
relatively late in stage4. My understanding is that at the moment there are no
direct plans to retpoline userland, but I see that it may change in future.
Can you give us bit of review if there are still some missing parts?
Thanks,
Honza