https://sourceware.org/bugzilla/show_bug.cgi?id=27433
Bug ID: 27433
Summary: RISC-V linker might lose relax opportunities since
"again" doesn't work as expected
Product: binutils
Version: unspecified
Status: NEW
Severity: enhancement
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: nelsonc1225 at sourceware dot org
Target Milestone: ---
We find that the more relax passes, the more chances of relaxations are
reduced. Originally, we set the boolean "again" to TRUE once the code is
actually deleted, and then we run the relaxations repeatedly if `again` is
still TRUE. But "again" only works for the relax pass itself, and won't affect
others. That is - we can not use "again" to re-run the entire relax passes
when we already enter into the following passes (cannot run the relax passes
backwards).
This is a minor enhancement, just that the commit
abd20cb637008da9d32018b4b03973e119388a0a will make the code size slightly
worse since the more relax passes.
Consider the following assembly case,
$ cat relax.s
.text
.global _start
_start:
lla a0, data_g
.rept 0x3fffe
nop
.endr
call _start
.option rvc
.align 2
add a0, a1, a2
.data
.global data_g
.dword 0x0
data_g:
.word 0x1000
$ riscv64-unknown-elf-as relax.s -o relax.o
$ riscv64-unknown-elf-ld --relax relax.o -o relax
$ riscv64-unknown-elf-objdump -d relax | grep -A 3 "auipc"
1100ac: fff00097 auipc ra,0xfff00
1100b0: 004080e7 jalr 4(ra) # 100b0 <_start>
1100b4: 00c58533 add a0,a1,a2
...
The "auipc + jalr" pattern is "call _start", and it should be relaxed to "jal"
since the range is at the boundary and enough.
Please see the details as follows,
https://sourceware.org/pipermail/binutils/2020-November/114223.html
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