On Tue, Nov 3, 2020 at 1:00 PM H.J. Lu <hjl.to...@gmail.com> wrote: > > On Tue, Nov 3, 2020 at 12:46 PM Jozef Lawrynowicz > <joze...@mittosystems.com> wrote: > > > > On Tue, Nov 03, 2020 at 11:58:04AM -0800, H.J. Lu via Gcc-patches wrote: > > > On Tue, Nov 3, 2020 at 10:22 AM Jozef Lawrynowicz > > > <joze...@mittosystems.com> wrote: > > > > > > > > On Tue, Nov 03, 2020 at 09:57:58AM -0800, H.J. Lu via Gcc-patches wrote: > > > > > On Tue, Nov 3, 2020 at 9:41 AM Jozef Lawrynowicz > > > > > <joze...@mittosystems.com> wrote: > > > > > > > > > > > > The attached patch implements TARGET_ASM_MARK_DECL_PRESERVED for > > > > > > ELF GNU > > > > > > OSABI targets, so that declarations that have the "used" attribute > > > > > > applied will be saved from linker garbage collection. > > > > > > > > > > > > TARGET_ASM_MARK_DECL_PRESERVED will emit an assembler ".retain" > > > > > > > > > > Can you use the "R" flag instead? > > > > > > > > > > > > > For the benefit of this mailing list, I have copied my response from the > > > > Binutils mailing list regarding this. > > > > The "comm_section" example I gave is actually innacurate, but you can > > > > see the examples of the variety of sections that would need to be > > > > handled by doing > > > > > > > > $ git grep -A2 "define.*SECTION_ASM_OP" gcc/ | grep "\".*\." > > > > > > > > > ... snip ... > > > > > Secondly, for seamless integration with the "used" attribute, we must > > > > > be > > > > > able to to mark the symbol with the used attribute applied as > > > > > "retained" > > > > > without changing its section name. For GCC "named" sections, this is > > > > > straightforward, but for "unnamed" sections it is a giant mess. > > > > > > > > > > The section name for a GCC "unnamed" section is not readily available, > > > > > instead a string which contains the full assembly code to switch to > > > > > one > > > > > of these text/data/bss/rodata/comm etc. sections is encoded in the > > > > > structure. > > > > > > > > > > Backends define the assembly code to switch to these sections (some > > > > > "*ASM_OP*" macro) in a variety of ways. For example, the unnamed > > > > > section > > > > > "comm_section", might correspond to a .bss section, or emit a .comm > > > > > directive. I even looked at trying to parse them to extract what the > > > > > name of a section will be, but it would be very messy and not robust. > > > > > > > > > > Meanwhile, having a .retain <symbol_name> directive is a very simmple > > > > > solution, and keeps the GCC implementation really concise (patch > > > > > attached). The assembler will know for sure what the section > > > > > containing > > > > > the symbol will be, and can apply the SHF_GNU_RETAIN flag directly. > > > > > > > > > > > Please take a look at > > > > > > https://gitlab.com/x86-gcc/gcc/-/commits/users/hjl/elf/shf_retain > > > > > > which is built in top of > > > > > > https://gcc.gnu.org/pipermail/gcc-patches/2020-February/539963.html > > > > > > I think SECTION2_RETAIN matches SHF_GNU_RETAIN well. If you > > > want, you extract my flags2 change and use it for SHF_GNU_RETAIN. > > > > In your patch you have to make the assumption that data_section, always > > corresponds to a section named .data. For just this example, c6x (which > > supports the GNU ELF OSABI) does not fit the rule: > > > > > c6x/elf-common.h:#define DATA_SECTION_ASM_OP > > > "\t.section\t\".fardata\",\"aw\"" > > > > data_section for c6x corresponds to .fardata, not .data. So the use of > > "used" on a data declaration would place it in a different section, that > > if the "used" attribute was not applied. > > > > For c6x and mips, readonly_data_section does not correspond to .rodata, > > so that assumption cannot be made either: > > > c6x/elf-common.h:#define READONLY_DATA_SECTION_ASM_OP > > > "\t.section\t\".const\",\"a\",@progbits" > > > mips/mips.h:#define READONLY_DATA_SECTION_ASM_OP "\t.rdata" > > > /* read-only data */ > > > > The same can be said for bss_section for c6x as well. > > Just add and use named_xxx_section. > > > Furthermore, this is only considering the examples in > > default_elf_select_section - the less standard unnamed section are used > > in many backend's implementation of select_section, and we would need to > > work out what section name they correspond to to properly support > > SHF_GNU_RETAIN. > > > > For every unnamed section, you either have to assume what the > > corresponding section name is, or parse the associated assembly output > > string for the section. > > My change is just an example to show how it can be done, not a complete one. > > > Given these edge cases which must be handled for GCC to robustly emit > > the "R" flag for sections containing "used" symbols, surely it is > > preferable to leverage the existing TARGET_ASM_MARK_DECL_PRESERVED and > > emit a .retain <symname> directive, which is extremely simple and > > doesn't require any handling of these edge cases and non-standard > > backend implementations. > > It is used to update the symbol table. Other usage is abuse. >
For [hjl@gnu-cfl-2 gcc]$ cat /tmp/x.c static int xyzzy __attribute__((__used__)) = 1; int foo[0x40000]; [hjl@gnu-cfl-2 gcc]$ foo should be removed by ld --gc-sections if it is unreferenced. But your patch makes it impossible. -- H.J.
