Re: [PATCH v1 2/4] MIPS: relocation: add relocation support

[Oleksij Rempel <linux@rempel-privat.de>]

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Am 04.03.19 um 20:05 schrieb Peter Mamonov:
> Hi, Oleksij,
> 
> On Mon, Feb 18, 2019 at 08:52:00AM +0100, Oleksij Rempel wrote:
>> From: Oleksij Rempel <linux@rempel-privat.de>
>>
>> this patch i a port of following patch from u-boot with some additional
>> integration changes and fixes of original code:
>> | Subject: [PATCH] MIPS: Stop building position independent code
>> |
>> | U-Boot has up until now built with -fpic for the MIPS architecture,
>> | producing position independent code which uses indirection through a
>> | global offset table, making relocation fairly straightforward as it
>> | simply involves patching up GOT entries.
>> |
>> | Using -fpic does however have some downsides. The biggest of these is
>> | that generated code is bloated in various ways. For example, function
>> | calls are indirected through the GOT & the t9 register:
>> |
>> |   8f998064   lw     t9,-32668(gp)
>> |   0320f809   jalr   t9
>> |
>> | Without -fpic the call is simply:
>> |
>> |   0f803f01   jal    be00fc04 <puts>
>> |
>> | This is more compact & faster (due to the lack of the load & the
>> | dependency the jump has on its result). It is also easier to read &
>> | debug because the disassembly shows what function is being called,
>> | rather than just an offset from gp which would then have to be looked up
>> | in the ELF to discover the target function.
>> |
>> | Another disadvantage of -fpic is that each function begins with a
>> | sequence to calculate the value of the gp register, for example:
>> |
>> |   3c1c0004   lui    gp,0x4
>> |   279c3384   addiu  gp,gp,13188
>> |   0399e021   addu   gp,gp,t9
>> |
>> | Without using -fpic this sequence no longer appears at the start of each
>> | function, reducing code size considerably.
>> |
>> | This patch switches U-Boot from building with -fpic to building with
>> | -fno-pic, in order to gain the benefits described above. The cost of
>> | this is an extra step during the build process to extract relocation
>> | data from the ELF & write it into a new .rel section in a compact
>> | format, plus the added complexity of dealing with multiple types of
>> | relocation rather than the single type that applied to the GOT. The
>> | benefit is smaller, cleaner, more debuggable code. The relocate_code()
>> | function is reimplemented in C to handle the new relocation scheme,
>> | which also makes it easier to read & debug.
>> |
>> | Taking maltael_defconfig as an example the size of u-boot.bin built
>> | using the Codescape MIPS 2016.05-06 toolchain (gcc 4.9.2, binutils
>> | 2.24.90) shrinks from 254KiB to 224KiB.
>> |
>> | Signed-off-by: Paul Burton <paul.burton@imgtec.com>
>>
>> Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
>> ---
>>
>> [...]
>>
>> diff --git a/arch/mips/Makefile b/arch/mips/Makefile
>> index 5fbd51ceee..5eccf18fe4 100644
>> --- a/arch/mips/Makefile
>> +++ b/arch/mips/Makefile
>> @@ -11,7 +11,7 @@ endif
>>  
>>  CPPFLAGS += -D__MIPS__ -fno-strict-aliasing -fno-merge-constants
>>  
>> -cflags-y += -G 0 -mno-abicalls -fno-pic -pipe
>> +cflags-y += -G 0 -mno-abicalls -pipe
> 
> The patch you've ported from das u-boot removes `-fpic` flag, yet your patch 
> removes `-fno-pic`, i.e. it does the opposite thing.

Good point, this part of the change has no influence. With or without this flag, noPIC code is
build.	I'll put it back to avoid confusion.


-- 
Regards,
Oleksij


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