[LLVMdev] RFC: ThinLTO Impementation Plan

[Duncan P. N. Exon Smith]

> On 2015-May-15, at 07:30, Teresa Johnson <tejohnson at google.com> wrote:
> 
>>> a. Clang/LLVM/gold linker options:
>>> 
>>> An early set of clang/llvm patches is needed to provide options to
>>> enable ThinLTO (off by default), so that the rest of the
>>> implementation can be disabled by default as it is added.
>>> Specifically, clang options -fthinlto (used instead of -flto) will
>>> cause clang to invoke the phase-1 emission of LLVM bitcode and
>>> function summary/index on a compile step, and pass the appropriate
>>> option to the gold plugin on a link step. The -thinlto option will be
>>> added to the gold plugin and llvm-lto tool to launch the phase-2 thin
>>> archive step. The -thinlto option will also be added to the ‘opt’ tool
>>> to invoke it as a phase-3 parallel backend instance.
>> 
>> I'm not sure I follow the `opt` part of this.  That's a developer
>> tool, not something we ship.  It also doesn't have a backend (doesn't
>> do CodeGen).  What am I missing?
> 
> For the prototype I was using llvm-lto as my backend driver. I
> realized that this was probably not the best option as we don't need
> all of the LTO handling built into that driver, and it isn't listed as
> a tool on http://llvm.org/docs/CommandGuide/, so my feeling was that
> 'opt' was better supported and a better alternative. Unfortunately
> when I was writing this up I forgot that 'opt' generates bitcode not
> an object file.

Both `opt` and `llvm-lto` are designed as tools for LLVM developers;
the latter in particular is just for testing LTO-related things without
needing to talk to a linker (so, it's platform agnostic).

Neither is supported for end users.

> 
> Another option would be to use clang and allow it to accept bitcode
> and bypass parsing under an appropriate ThinLTO option. AFAICT there
> isn't currently an option for clang to accept bitcode. Do you think
> this is the right approach?

Clang handles bitcode just fine.  The `-save-temps` option even
round-trips through bitcode:

$ lrun -w staging clang t.c -save-temps -###
/Users/dexonsmith/data/llvm.symbols+asserts/staging/bin/clang t.c -save-temps -###
clang version 3.7.0 (trunk 236655) (llvm/trunk 236658)
Target: x86_64-apple-darwin14.4.0
Thread model: posix
 "/Volumes/Data/llvm.symbols+asserts/staging/bin/clang-3.7" "-cc1" "-triple" "x86_64-apple-macosx10.10.0" "-E" "-disable-free" "-main-file-name" "t.c" "-mrelocation-model" "pic" "-pic-level" "2" "-mthread-model" "posix" "-mdisable-fp-elim" "-masm-verbose" "-munwind-tables" "-target-cpu" "core2" "-target-linker-version" "250" "-dwarf-column-info" "-fno-unique-section-names" "-resource-dir" "/Volumes/Data/llvm.symbols+asserts/staging/bin/../lib/clang/3.7.0" "-fdebug-compilation-dir" "/tmp" "-ferror-limit" "19" "-fmessage-length" "160" "-stack-protector" "1" "-mstackrealign" "-fblocks" "-fobjc-runtime=macosx-10.10.0" "-fencode-extended-block-signature" "-fmax-type-align=16" "-fdiagnostics-show-option" "-fcolor-diagnostics" "-o" "t.i" "-x" "c" "t.c"
 "/Volumes/Data/llvm.symbols+asserts/staging/bin/clang-3.7" "-cc1" "-triple" "x86_64-apple-macosx10.10.0" "-emit-llvm-bc" "-emit-llvm-uselists" "-disable-free" "-main-file-name" "t.c" "-mrelocation-model" "pic" "-pic-level" "2" "-mthread-model" "posix" "-mdisable-fp-elim" "-masm-verbose" "-munwind-tables" "-target-cpu" "core2" "-target-linker-version" "250" "-dwarf-column-info" "-fno-unique-section-names" "-resource-dir" "/Volumes/Data/llvm.symbols+asserts/staging/bin/../lib/clang/3.7.0" "-fdebug-compilation-dir" "/tmp" "-ferror-limit" "19" "-fmessage-length" "160" "-stack-protector" "1" "-mstackrealign" "-fblocks" "-fobjc-runtime=macosx-10.10.0" "-fencode-extended-block-signature" "-fmax-type-align=16" "-fdiagnostics-show-option" "-fcolor-diagnostics" "-disable-llvm-optzns" "-o" "t.bc" "-x" "cpp-output" "t.i"
 "/Volumes/Data/llvm.symbols+asserts/staging/bin/clang-3.7" "-cc1" "-triple" "x86_64-apple-macosx10.10.0" "-S" "-disable-free" "-main-file-name" "t.c" "-mrelocation-model" "pic" "-pic-level" "2" "-mthread-model" "posix" "-mdisable-fp-elim" "-masm-verbose" "-munwind-tables" "-target-cpu" "core2" "-target-linker-version" "250" "-dwarf-column-info" "-fno-unique-section-names" "-resource-dir" "/Volumes/Data/llvm.symbols+asserts/staging/bin/../lib/clang/3.7.0" "-fdebug-compilation-dir" "/tmp" "-ferror-limit" "19" "-fmessage-length" "160" "-stack-protector" "1" "-mstackrealign" "-fblocks" "-fobjc-runtime=macosx-10.10.0" "-fencode-extended-block-signature" "-fmax-type-align=16" "-fdiagnostics-show-option" "-fcolor-diagnostics" "-o" "t.s" "-x" "ir" "t.bc"
 "/Volumes/Data/llvm.symbols+asserts/staging/bin/clang-3.7" "-cc1as" "-triple" "x86_64-apple-macosx10.10.0" "-filetype" "obj" "-main-file-name" "t.c" "-target-cpu" "core2" "-mrelax-all" "-o" "t.o" "t.s"
 "/usr/bin/ld" "-demangle" "-dynamic" "-arch" "x86_64" "-macosx_version_min" "10.10.0" "-o" "a.out" "t.o" "-lSystem"