[patch][rfc] Implicit sections

[Rafael Espíndola]

The linker in the ps4 sdk is able to split sections into smaller
chunks for GC. It does so using the symbol size that is recorded in
the ELF files (unlike MachO that uses the next symbol to find the
boundary).

As part of the work of upstreaming the llvm changes, I have been
looking at a way to view and implement this as a general and sound
optimization.

In order to be able to split sections, the linker needs extra
relocations. In fact, it needs the same relocations as would be needed
if -ffunction-sections and -fdata-sections were used.

This led to the insight that a way to view this is just as a more
compact .o representation of multiple sections. The codegen and
assembler work as if -function-sections/-fdata-sections were used, but
we can skip outputting the extra sections because we know that the
linker can recreate them.

The wip patch of the first step is attached. What it does is add
support for multiple sections with the same name. It does so by adding
a "unique" marker (bike sheds on the name welcome). This just means
that every .section with the unique marker in the .s creates a new
section.

With the support in place, the patch then avoids creating multiple
section names when using -ffunction-section and -fdata-section. The .o
files just end up with many .text sections (for example).

This should work with every ELF linker.

The patch needs a bit more work (proper option instead of cl::opt,
more compact encoding), but it can bootstrap and saves some space in
the .o files. In a build dir after running "ninja && ninja check-all"
the total size of the .o files goes from 237871087 bytes to 231292287.

The patch also offers some saving even when
-ffunction-sections/-fdata-sections are not used by avoiding long
names in comdats.

The next step would then be to add an "implicit" flag in addition to
the "unique" one. This would mean that when given

.section .text,"ax", at progbits, implicit
.section .text,"ax", at progbits, implicit

MC would in the end output a single .text section, but would produce
the extra relocation in order to make it possible to split them in the
linker.

The resulting file would still be a legal ELF, it would just require
extra logic in the linker to be able to GC it. The tradeoff being the
smaller .o files.

I still don't have the numbers on what the saving from omitting the
sections might be, but a as an upper bound, in a build with clang
using this patch and no -ffunction-section/-fdata-sections the total
.o size is 217519383 bytes.

With this in then the default for PS4 would be changed to
-ffunction-sections, -fdata-sections and implicit sections.

The advantage of implementing it this way is that it makes it clear
that it is really just a .o compactness optimization. No extra logic
is required for deciding if a relocation is needed. The extra handling
of "implicit" would be just in the ELF object writer.

Cheers,
Rafael
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