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[PATCH v2] Support for SHF_GNU_RETAIN ELF Section Flag

H.J. Lu hjl.tools@gmail.com
Wed Sep 30 22:13:15 GMT 2020 
On Mon, Sep 28, 2020 at 6:28 AM Jozef Lawrynowicz
<jozef.l@mittosystems.com> wrote:
>
> The attached patch is version 2 of the SHF_GNU_RETAIN patch that was
> previously discussed here:
> https://sourceware.org/pipermail/binutils/2020-September/113406.html
>
> The following changes have been made:
> - Removed the .retain directive
> - The assembler will create different input sections for sections with
>   the same name but SHF_GNU_RETAIN set/unset (thanks to H.J. for that).
>   This means the linker will be able to do a better job garbage
>   collecting input sections, as the "retain" attribute applied to a
>   symbol declaration in the source code will not cause other parts of
>   the program that are not required, but are in the same section, to be
>   unnecessarily retained.
> - Added GNU OSABI handling (also thanks to H.J.).
>
> I've successfully regtested the patch for the Binutils, GAS and LD
> testsuites for the following targets:
>
> aarch64-elf arc-elf arm-eabi arm-elf avr-elf bfin-elf cr16-elf cris-elf
> crx-elf csky-elf d10v-elf d30v-elf dlx-elf epiphany-elf fr30-elf frv-elf
> ft32-elf h8300-elf hppa-elf i386-elf ip2k-elf iq2000-elf lm32-elf
> m32c-elf m32r-elf m68hc11-elf m68hc12-elf m68k-elf mcore-elf mep-elf
> metag-elf microblaze-elf mips-elf moxie-elf msp430-elf mt-elf nios2-elf
> or1k-elf pj-elf ppc-elf pru-elf riscv-elf rl78-elf rx-elf s12z-elf
> score-elf sh-elf sparc-elf spu-elf tic6x-elf tilegx-elf tilepro-elf
> v850-elf visium-elf wasm32-elf xgate-elf xstormy16-elf xtensa-elf
> z80-elf.
>
> The new tests only run for the targets that use the GNU OSABI. From the
> above list, arm-elf, hppa-elf, msp430-elf, visium-elf do not use the GNU
> OSABI, so do not support SHF_GNU_RETAIN.
>
> Ok to apply?
>
> Thanks,
> Jozef

+# SHF_GNU_RETAIN requires the GNU OSABI.
+if { [check_gc_sections_available] && [supports_gnu_osabi] } {
+    run_ld_link_tests [list \
+ [list "SHF_GNU_RETAIN 1" \
+     "--gc-sections -e _start --print-gc-sections" "" "" \
+     {retain1.s} \
+     {{ ld retain1.msg }} \
+     "retain1.exe"] \
+ [list "SHF_GNU_RETAIN 3 (keep sections referenced by retained sections)" \
+     "--gc-sections -e _start --print-gc-sections" "" "" \
+     {retain3.s} {} \
+     "retain3.exe"] \
+ [list "SHF_GNU_RETAIN 4 (keep orphaned sections when not discarding)" \
+     "--gc-sections -e _start --print-gc-sections
--orphan-handling=place" "" "" \
+     {retain4.s} {} \
+     "retain4.exe"] \
+ [list "Build libretain5.a" \
+     "" "" "" \
+     {retain5lib.s} {} "libretain5.a"] \
+ [list "SHF_GNU_RETAIN 5 (don't pull SHF_GNU_RETAIN section out of lib)" \
+     "--gc-sections -e _start --print-gc-sections" "-Ltmpdir -lretain5" "" \
+     {retain5main.s} {} \
+     "retain5.exe"] \
+ [list "Build libretain6.a" \
+     "" "" "" \
+     {retain6lib.s} {} "libretain6.a"] \
+ [list "SHF_GNU_RETAIN 6 (pull section out of lib required by
SHF_GNU_RETAIN section)" \
+     "--gc-sections -e _start --print-gc-sections" "-Ltmpdir -lretain6" "" \
+     {retain6main.s} \
+     {{ ld retain6.msg }} \
+     "retain6.exe"] \
+ ]
+}
+

Can you use *.d files like

retain6.d:
#ld: --gc-sections --print-gc-sections -e _start
#warning_output: retain6.l
#target: [check_gc_sections_available]

instead?

-- 
H.J.
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