[ Memory consumption ] PC-relative addresses instead of relocations in code section on ARM32

[ruben-ayrapetyan]

Hello.

We propose to reduce consumption of memory, which currently occurs due to applying relocations to code section of ngen-ed images.

To achieve this, we propose to replace movw+movt relocations with movw+movt+add pc instruction sequence. In the second case, the movw+movt encodes offset, which is relative to instruction position, so applying relocations becomes unnecessary.

We plan to add JitFlags::JIT_FLAG_RELATIVE_CODE_RELOCS as switch for relocated/position-independent mode for code section.

What do you think about enabling the option by default for ARM32?

Estimation on Tizen Xamarin GUI Application (specifically, Puzzle.Tizen - see tables at #7694 (comment)) is about 1 megabytes (15%) savings of per-application CoreCLR's memory part in Fragile mode.

@lemmaa @egavrin @Dmitri-Botcharnikov @gbalykov

[ruben-ayrapetyan]

@jkotas, Please take a look.

[ruben-ayrapetyan]

For now, we implemented prototype for the changes.
We plan to open pull request with complete patch this week.

[jkotas]

What do you think about enabling the option by default for ARM32?

I think it make sense for Linux ARM32 native images. I do not think it makes sense for Windows ARM32. The relocations are handled by the OS kernel on Windows ARM32 that makes them cheap.

cc @BruceForstall

[ruben-ayrapetyan]

@jkotas, Thank you.