Re: [RFC] introduce --param max-lto-partition for having an upper bound on partition size

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

On 1 April 2016 at 23:02, Richard Biener <rguenther@suse.de> wrote:
> On April 1, 2016 3:48:35 PM GMT+02:00, Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org> wrote:
>>Hi,
>>The attached patch introduces param max-lto-partition which creates an
>>upper
>>bound for partition size.
>>
>>My primary motivation for this patch is to fix building chromium for
>>arm
>>with -flto-partition=one.
>>Chromium fails to build with -flto-partition={none, one} with assembler
>>error:
>>"branch out of range error"
>>because in both these cases LTO creates a single text section of 18 mb
>>which exceeds thumb's limit of 16 mb and arm backend emits a short
>>call if caller and callee are in same section.
>>This is binutils PR18625:
>>https://sourceware.org/bugzilla/show_bug.cgi?id=18625
>>With patch, chromium builds for -flto-partition=one (by creating more
>>than one but minimal number of partitions to honor 16 mb limit).
>>I haven't tested with -flto-partition=none but I suppose the build
>>will still fail for none, because it won't involve partitioning?  I am
>>not sure how to fix for none case.
>>
>>As suggested by Jim in binutils PR18625, the proper fix would be to
>>implement branch relaxation in arm's port of gas, however I suppose
>>only LTO will realistically create such large sections,
>>and implementing branch relaxation appears to be quite complicated and
>>probably too much of
>>an effort for this single use case, so this patch serves as a
>>work-around to the issue.
>>I am looking into fine-tuning the param value for ARM backend to
>>roughly match limit
>>of 16 mb.
>>
>>AFAIU, this would change semantics of --param n_lto_partitions (or
>>-flto-partition=one) from
>>"exactly n_lto_partitions" to "at-least n_lto_partitions". If that's
>>not desirable maybe we could add
>>another param/option ?
>>Cross-tested on arm*-*-*.
>>Would this patch be OK for stage-1 (after getting param value right
>>for ARM target) ?
>
> What do you want to achieve?  Changing =one semantics doesn't look right to me.
> Adding a param for maximum size sounds good in general, but only to increase the maximum number of partitions for =balanced (the default).
Well, chromium fails to build on ARM with -flto-partition={none, one}
because the size of text section created with LTO,
exceeds the limit of 16 mb for thumb2 which results in assembler
errors: "branch out of range".
I was trying to fix that by creating minimal number of partitions such
that size of each partition is not greater than section size limit.
I suppose in theory the problem could also present with balanced
partitioning if total_size / n_lto_partitions exceeds section size
limit,
although not sure if this will be a practical case.

Thanks,
Prathamesh
>
> Richard.
>
>>
>>Thanks,
>>Prathamesh
>
>