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Re: [PATCH] Expand PIC calls without PLT with -fno-plt
- From: Jan Hubicka <hubicka at ucw dot cz>
- To: Alexander Monakov <amonakov at ispras dot ru>
- Cc: gcc-patches at gcc dot gnu dot org, Rich Felker <dalias at libc dot org>
- Date: Sun, 10 May 2015 18:59:44 +0200
- Subject: Re: [PATCH] Expand PIC calls without PLT with -fno-plt
- Authentication-results: sourceware.org; auth=none
- References: <1430757479-14241-1-git-send-email-amonakov at ispras dot ru> <1430757479-14241-6-git-send-email-amonakov at ispras dot ru>
> This patch introduces option -fno-plt that allows to expand calls that would
> go via PLT to load the address of the function immediately at call site (which
> introduces a GOT load). Cover letter explains the motivation for this patch.
>
> New option documentation for invoke.texi is missing from the patch; if this is
> accepted I'll be happy to send a v2 with documentation added.
>
> * calls.c (prepare_call_address): Transform PLT call to GOT lookup and
> indirect call by forcing address into a pseudo with -fno-plt.
> * common.opt (flag_plt): New option.
>
> diff --git a/gcc/common.opt b/gcc/common.opt
> index b49ac46..cd8b256 100644
> --- a/gcc/common.opt
> +++ b/gcc/common.opt
> @@ -1773,12 +1773,16 @@ Common Report Var(flag_pic,1) Negative(fpie)
> Generate position-independent code if possible (small mode)
>
> fpie
> Common Report Var(flag_pie,1) Negative(fPIC)
> Generate position-independent code for executables if possible (small mode)
>
> +fplt
> +Common Report Var(flag_plt) Init(1)
> +Use PLT for PIC calls (-fno-plt: load the address from GOT at call site)
> +
This won't play well with LTO since fplt will become another global flag while
it affects codegen.
I still did not catch up with the other thread and Hj's work on doing this
transparently in linker, but if this is getting in, please add Optimization to
fplt, so the PLT usage can be decided with per function granuality.
Honza