[PATCH 1/2] Gold: Skip zero-sized sections for ICF

[Fangrui Song]

On 2020-10-11, H.J. Lu wrote:
>On Sun, Oct 11, 2020 at 5:02 PM Fangrui Song <i@maskray.me> wrote:
>>
>> On 2020-10-11, H.J. Lu via Binutils wrote:
>> >Skip zero-sized sections since there is no need to do ICF on them.
>> >
>> >       * icf.cc (Icf::find_identical_sections): Skip zero-sized sections.
>> >---
>> > gold/icf.cc | 2 ++
>> > 1 file changed, 2 insertions(+)
>> >
>> >diff --git a/gold/icf.cc b/gold/icf.cc
>> >index a60db7abc8d..54af4126696 100644
>> >--- a/gold/icf.cc
>> >+++ b/gold/icf.cc
>> >@@ -973,6 +973,8 @@ Icf::find_identical_sections(const Input_objects* input_objects,
>> >
>> >       for (unsigned int i = 0; i < (*p)->shnum(); ++i)
>> >         {
>> >+          if ((*p)->section_size(i) == 0)
>> >+            continue;
>> >         const std::string section_name = (*p)->section_name(i);
>> >           if (!is_section_foldable_candidate(section_name))
>> >           {
>> >--
>> >2.26.2
>> >
>>
>> Does anything break without the special case? ld.lld --icf={all,safe} does not
>> special case st_size=0
>>
>>
>> .section .text.1,"ax"; .globl foo; foo:
>> .section .text.2,"ax"; .globl bar; bar:
>>
>> % ld.lld a.o --icf=all --print-icf-sections
>> selected section a.o:(.text)
>>    removing identical section a.o:(.text.1)
>>    removing identical section a.o:(.text.2)
>> ld.lld: warning: cannot find entry symbol _start; defaulting to 0x201158
>
>The zero-sized input functions should have no impact on output,
>ICF or not.

Folding a zero-sized section has no benefit, so we can either do or not
do it. Since not doing it adds 2 lines of code, I'd rather we do it.