Re: [PATCH: PR/43129] Simplify global variable's address loading with option -fpic

[Carrot Wei <carrot at google dot com>]

Ping?

On Tue, Apr 13, 2010 at 10:49 PM, Carrot Wei <carrot@google.com> wrote:
> Hi
>
> The bug http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43129 describes how arm
> backend generates ineffecient code to load global variable's address in terms
> of both code size and performance when option -fpic is specified. It also
> describes how it can be improved by relocation R_ARM_GOT_PREL.
>
> This patch contains a new pass simplify-got to implement the suggested
> optimization. It is placed between gcse and ira.
>
> This new pass can also be applied to different targets when they satisfy
>
> 1. There should be at least 2 methods to load a global variable's address from
> GOT. Usually it means using different relocation types.
>
> 2. By default all global variable's address loading use the same method: first
> load the base address of GOT into a register pic_reg, then the real global
> variable's address is loaded as
> ? ? ? ? ? (SET offset_reg <the offset from GOT base to the GOT entry>)
> ? ? ? ? ? (SET address_reg (MEM (PLUS pic_reg offset_reg)))
>
> 3. If the number of global address loading is less than a target specific
> threshold the alternative method may be better.
>
> 4. The alternative method doesn't use the base of GOT (pic_reg).
>
> This patch depends on the arm relocation R_ARM_GOT_PREL which is only
> implemented in mainline binutils. So this patch won't be check in until the
> next major release of binutils.
>
>
> Test:
> x86_64 bootstrap.
> arm regression test on qemu.
> build CSiBE for arm with options "-Os -fpic -mthumb -fno-short-enums",
> the total size is:
> without simplify-got ? ?2830665
> with simplify-got ? ? ? 2825737
>
> ChangeLog:
>
> 2010-04-13 ?Wei Guozhi ?<carrot@google.com>
>
> ? ? ? ?PR/43129
> ? ? ? ?tree-pass.h (pass_simplify_got): New variable declaration.
> ? ? ? ?timevar.def (TV_SIMPLIFY_GOT): New timevar id.
> ? ? ? ?passes.c (init_optimization_passes): Add the new pass.
> ? ? ? ?simplify-got.c : New file implement simplify-got pass.
> ? ? ? ?Makefile.in (simplify-got.o): New object.
> ? ? ? ?hooks.h (hook_rtx_void_null): New function declaration.
> ? ? ? ?hooks.c (hook_rtx_void_null): New function.
> ? ? ? ?target.h (simplify_got_access): New structure contains simplify-got
> ? ? ? ? ? ? ? ?related hooks.
> ? ? ? ?target-def.h (TARGET_GET_PIC_REG): Define hook TARGET_GET_PIC_REG.
> ? ? ? ?(TARGET_CLEAR_PIC_REG): Likewise.
> ? ? ? ?(TARGET_LOADED_GLOBAL_VAR): Likewise.
> ? ? ? ?(TARGET_MAX_USAGE_TO_AVOID_PIC_REG): Likewise.
> ? ? ? ?(TARGET_LOAD_GLOBAL_ADDRESS): Likewise.
> ? ? ? ?doc/tm.texi (TARGET_GET_PIC_REG): Document this new hook.
> ? ? ? ?(TARGET_CLEAR_PIC_REG): Likewise.
> ? ? ? ?(TARGET_LOADED_GLOBAL_VAR): Likewise.
> ? ? ? ?(TARGET_MAX_USAGE_TO_AVOID_PIC_REG): Likewise.
> ? ? ? ?(TARGET_LOAD_GLOBAL_ADDRESS): Likewise.
> ? ? ? ?config/arm/arm.md (UNSPEC_GOT_PREL_SYM): New unspec value.
> ? ? ? ?config/arm/arm.c (arm_get_pic_reg): New function.
> ? ? ? ?(arm_clear_pic_reg): New function.
> ? ? ? ?(arm_max_usage_to_avoid_pic_reg): New function.
> ? ? ? ?(arm_loaded_global_var): New function.
> ? ? ? ?(arm_load_global_address): New function.
> ? ? ? ?(arm_output_addr_const_extra): Output const expression for
> ? ? ? ?new relocation R_ARM_GOT_PREL.
>
> 2010-04-13 ?Wei Guozhi ?<carrot@google.com>
>
> ? ? ? ?PR/43129
> ? ? ? ?gcc.target/arm/got1.c: New testcase.
> ? ? ? ?gcc.target/arm/got2.c: New testcase.
>
>
> thanks
> Wei Guozhi
>