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Subject: Re: [PATCH] Give --gc-sections to the linker

Re: [PATCH] Give --gc-sections to the linker

From: Boris Gjenero <boris.gjenero_at_gmail.com>
Date: Sun, 04 Dec 2011 23:01:34 -0500

On 04/12/11 06:23 PM, Rafaël Carré wrote:
> flto at least failed to work when mixing thumb and arm code (i think
> the last version i tried was gcc 4.6.1)

I didn't spend much time on arm -flto, because I was mainly interested
in getting smaller Archos binaries. I didn't even get a working 5G iPod
binary with binutils 2.21.1 and the normal compiler. It seems like the
majority of binutils-2.20.1-ld-thumb-interwork-long-call.diff is in
2.21.1, but it's not an exact match. I wonder if that is the problem.

I wanted to use the new binutils because the link time optimizer needs
linker plugin support to access files in archives (like libfirmware.a).
A compile can succeed without that, but it's not fully link time optimized.

> In the long run it would be nice to use it though, if properly tested
> (especially wrt codecs speed).

If the best possible codec speed is desired, it might make sense to use
per-codec optimization options. For example -flto could be used for some
but not for others. I'm not sure if this could get too messy and
difficult if different targets need different settings.

>>> I can see some linker scripts have something like that already:
>>> KEEP(*(.vectors));// otherwise there are no references to it and the linker strip it
>>
>> Yes. For example, the firmware/target/sh/archos/app.lds has it, and so
>> -Wl,--gc-sections can be used without problems on Archos hwcodec targets.
>
> Should we mark all sections as KEEP in all scripts ?

No, just the ones that are necessary but not referenced by other
sections. That should just be the vectors. They're for use by the CPU,
and not accessed by anything in Rockbox. I expect putting KEEP
everywhere would defeat --gc-sections.

BTW. -flto has its own similar problem. When compiling for the Recorder
V2, gcc 4.6.2 failed to figure out that C code referenced from assembly
code needed to be kept. In most cases, that's an undefined reference,
but the interrupt vector table uses .weak to make vectors default to
their UIE location. All the interrupt handlers got discarded, and every
interrupt led to UIE(). __attribute__((used)) fixed that.

The other -flto problems had to do with how code gets shuffled. It's not
possible to assume that C code stays within range of a relative branch,
and assembler code labels need to have .global because C functions in
the same file can end up in a different assembler file.

Regards,

Boris
Received on 2011-12-05


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