Re: global pointer gets overwritten with dlopen(3) on RISC-V

[Fangrui Song <maskray@google.com>]

On 2023-05-12, Palmer Dabbelt wrote:
>On Fri, 12 May 2023 13:11:43 PDT (-0700), fweimer@redhat.com wrote:
>>* Fangrui Song:
>>
>>>>1. Make -mno-relax the default for ld(1) (on Linux?). We have no
>>>>benchmarks whatsoever, but global variables aren't very popular in
>>>>application code these days and the gp register allows access to a
>>>>single memory page (4kB) only. No big deal really.
>>>
>>>I do agree that --no-relax-gp is a sensible default choice for GNU ld.
>>>https://maskray.me/blog/2021-03-14-the-dark-side-of-riscv-linker-relaxation#global-pointer-relaxation
>>>
>>>Perhaps you can start a separate topic on binutils? :)
>>>
>>>According to a doc from SiFive about -static -mcpu=sifive-u74 builds,
>>>https://docs.google.com/spreadsheets/d/14V7cPbyc80AcGHzsMaw9hYb232dzRbGCmTApnxj-SpU/edit#gid=0
>>>global pointer relaxation saves at best 0.5% size (I guess that refers
>>>to .text. If we count all allocable sections, the percentage is likely
>>>even smaller.)
>>
>>For a mature toolchain, 0.5% in code size reduction would be *a lot*,
>>so I wouldn't dismiss that.
>
>That's broadly speaking why it sticks around.  We've got a bunch of 
>headaches related to relaxation, GP or otherwise, but they improve 
>performance and nobody's figured out how to replace that yet.
>
>>Do we have a reproducer?  Is the issue actually about gp relaxation for
>>the main executable?
>
>In general we don't reference GP from shared libraries as we don't 
>have a GP save/restore scheme.  There may be a bug floating around 
>here somewhere, in which case we should fix it, but the original post 
>sounds like it wasn't a supported use case.
>
>>Thanks,
>>Florian

Global pointer relaxation only applies to +-2KiB data relative to __global_pointer$ (= .sdata + 0x800).
The area that potentially benefits global pointer relaxation is very small.

0.5% code size reduction (relative to .text?) is the best case. I
suspect the program somehow has a lot global variable accesses and
placing these variables in .sdata helps.

I've got results from Yingwei Zheng at PLCT lab using many
configurations. The saving is like 0.1%.
https://docs.google.com/spreadsheets/d/1Gz0h-C4U0toa9qELFtRaEWT_CzauE5JD9xMsLR8RyK8/edit#gid=1721258109

On the binutils side, we occasionally see patches to fix global pointer
relaxation bugs, e.g. the patch just sent a few hours ago:
https://sourceware.org/pipermail/binutils/2023-May/127413.html

I do not know the embedded toolchain well, but for Linux desktop/server,
disabling global pointer relaxation seems like a sensible choice. If we
discover a better way to utilize GP (x3) in the future, disabling global
pointer relaxation today will result in fewer compatibility issues.

Haiku
(https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/298#issuecomment-1344724796), Android, and Fuchsia
have mentioned that they don't use global pointer relaxation.