[llvm-dev] [LLD] Linker Relaxation

[Bruce Hoult via llvm-dev]

On Wed, Jul 12, 2017 at 10:26 AM, PkmX via llvm-dev <llvm-dev at lists.llvm.org
> wrote:

> Note that RISC-V also handles alignment as part of relaxation, so it
> isn't really optional. For example:
>
> _start:
>     mv      a0, a0
>     .p2align 2
>     li      a0, 0
>
> The assembler inserts a 3-byte padding (note: this behavior isn't
> merged yet, see: https://github.com/riscv/riscv-binutils-gdb/pull/88):
>
> 00000000 <_start>:
>    0:   852a                    mv      a0,a0
>    2:   00 01 00                        # R_RISCV_ALIGN
>                         2: R_RISCV_ALIGN        *ABS*+0x3
>    5:   4501                    li      a0,0
>
> The linker then remove 1 byte from padding to align to the desired width:
>

This seems ... pessimised :-)

At least output 4 bytes not 3! To keep it legal before the linker changes
sizes.

The assembler can easily keep track of the current alignment as it
generates the code, and create an object file that is correct assuming no
size changes by the linker.

If there is, say, a .byte literal of length one then the ".p2align 2"
should perhaps emit 7 bytes? Then it is legal as-is if the linker doesn't
change the size of anything, or the linker can delete as much as possible
if it is adjusting sizes.
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