ELF output contains lots of zero data: linker script issue? about riscv-rt HOT 4 CLOSED

FWIW this is on K210 and all memory regions are simply aliased to SRAM, sections are not treated any differently there:

MEMORY
{
    SRAM : ORIGIN = 0x80000000, LENGTH = 6M
…
}

REGION_ALIAS("REGION_TEXT", SRAM);
REGION_ALIAS("REGION_RODATA", SRAM);
REGION_ALIAS("REGION_DATA", SRAM);
REGION_ALIAS("REGION_BSS", SRAM);
REGION_ALIAS("REGION_HEAP", SRAM);
REGION_ALIAS("REGION_STACK", SRAM);

from riscv-rt.

To be honest, I'm not good at writing linker scripts and almost sure we can do better. However, I have no idea how to achieve this. The output you see is probably due to the .stack section existence, and I added this section to be able to check for the total stack size like this:

from riscv-rt.

Pretty sure that that should work.

I looked into it a bit yesterday (even as far as looking into the lld ELF generation code) and I'm baffled. From everything I saw, (NOLOAD) should be enough to get both no PT_LOAD flag a SHT_NOBITS section, the same combination as .bss. However while PT_LOAD is successfully disabled (so this doesn't actually affect anything),, it is a SHT_PROGBITS section. This combination is sometimes used for metadata and such. But it's obviously not what we want here.

So I'm starting to think it could be some lld issue. But not sure.

Separately from this, we could consider making the section only as big as needed instead of cover all of memory. Make it start AT((_stack_start - _hart_stack_size, * (_max_hart_id + 1)) and be exactly _hart_stack_size, * (_max_hart_id + 1) bytes large.
But I'm not sure that can be expressed.

from riscv-rt.

This seems to be a known (and fixed) issue: rust-lang/rust#73201
Introduced in nightly-2020-05-22 by LLVM update rust-lang/rust#67759, fixed in nightly-2020-06-08 by rust-lang/rust#73072
Present in stable 1.45.0 and 1.45.1, fixed in beta 1.46.0.

from riscv-rt.