[llvm-dev] A Propeller link (similar to a Thin Link as used by ThinLTO)?

Mehdi AMINI via llvm-dev llvm-dev at lists.llvm.org
Mon Mar 2 23:55:22 PST 2020

On Thu, Feb 27, 2020 at 6:34 PM Fangrui Song via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I met with the Propeller team today (we work for the same company but it
> was my first time meeting two members on the team:) ).
> One thing I have been reassured:
> * There is no general disassembly work. General
> disassembly work would assuredly frighten off developers.  (Inherently
> unreliable, memory usage heavy and difficult to deal with CFI, debug
> information, etc)
> Minimal amount of plumbing work (https://reviews.llvm.org/D68065) is
> acceptable: locating the jump relocation, detecting the jump type,
> inverting the direction of a jump, and deleting trailing bytes of an
> input section

. The existing linker relaxation schemes already do similar
> things. Deleting a trailing jump is similar to RISC-V where sections can
> shrink (not implemented in lld; R_RISCV_ALIGN and R_RISCV_RELAX are in
> my mind)) (binutils supports deleting bytes for a few other
> architectures, e.g.  msp430, sh, mips, ft32, rl78).  With just minimal
> amount of disassembly work, conceptually the framework should not be too
> hard to be ported to another target.
> One thing I was not aware of (perhaps the description did not make it
> clear) is that
> Propeller intends to **reorder basic block sections across translation
> units**.
> This is something that full LTO can do while ThinLTO cannot.
> Our internal systems cannot afford doing a full LTO (**Can we fix the
> bottleneck of full LTO** [1]?)
> for large executables and I believe some other users are in the same camp.

Right, beyond distributed build system, even on a single machine and for
"small" projects like clang: building on a laptop with FullLTO can be
challenging in terms of memory consumption, and the iterative development
is just not practical.

> Now, with ThinLTO, the post link optimization scheme will inevitably
> require
> help from the linker/compiler. It seems we have two routes:
> ## Route 1: Current Propeller framework
> lld does whole-program reordering of basic block sections.  We can extend
> it in
> the future to overalign some sections and pad gaps with NOPs.  What else
> can we
> do? Source code/IR/MCInst is lost at this stage. Without general assembly
> work, it may be difficult to do more optimization.
> This makes me concerned of another thing: Intel's Jump Condition Code
> Erratum.
> https://www.intel.com/content/dam/support/us/en/documents/processors/mitigations-jump-conditional-code-erratum.pdf
> Put it in the simplest way, a Jcc instruction whose address ≡ 30 or 31
> (mod 32) should be avoided.  There are assembler level (MC) mitigations
> (function sections are overaligned to 32), but because we use basic
> block sections (sh_addralign<32) and need reordering, we have to redo
> some work at the linking stage.
> After losing the representation of MCInst, it is not clear to me how we can
> insert NOPs/segment override prefixes without doing disassembly work in
> the linker.
> Route 2 does heavy lifting work in the compiler, which can naturally reuse
> the assembler level mitigation,
> CFI and debug information generating, and probably other stuff.
> (How will debug information be bloated?)
> ## Route 2: Add another link stage, similar to a Thin Link as used by
> ThinLTO.
> Regular ThinLTO with minimized bitcode files:
>         all: compile thin_link thinlto_backend final_link
>         compile a.o b.o a.indexing.o b.indexing.o: a.c b.c
>                 $(clang) -O2 -c -flto=thin
> -fthin-link-bitcode=a.indexing.o a.c
>                 $(clang) -O2 -c -flto=thin
> -fthin-link-bitcode=b.indexing.o b.c
>         thin_link lto/a.o.thinlto.bc lto/b.o.thinlto.bc a.rsp:
> a.indexing.o b.indexing.o
>                 $(clang) -fuse-ld=lld -Wl,--thinlto-index-only=a.rsp
> -Wl,--thinlto-prefix-replace=';lto'
> -Wl,--thinlto-object-suffix-replace='.indexing.o;.o' a.indexing.o
> b.indexing.o
>         thinlto_backend lto/a.o lto/b.o: a.o b.o lto/a.o.thinlto.bc
> lto/b.o.thinlto.bc
>                 $(clang) -O2 -c -fthinlto-index=lto/a.o.thinlto.bc a.o -o
> lto/a.o
>                 $(clang) -O2 -c -fthinlto-index=lto/b.o.thinlto.bc b.o -o
> lto/b.o
>         final_link exe: lto/a.o lto/b.o a.rsp
>                 # Propeller does basic block section reordering here.
>                 $(clang) -fuse-ld=lld @a.rsp -o exe
> We need to replace the two stages thinlto_backend and final_link with
> three.
> Propelled ThinLTO with minimized bitcode files:
>         propelled_thinlto_backend lto/a.mir lto/b.mir: a.o b.o
> lto/a.o.thinlto.bc lto/b.o.thinlto.bc
>                 # Propeller emits something similar to a Machine IR file.
>                 # a.o and b.o are all IR files.
>                 $(clang) -O2 -c -fthinlto-index=lto/a.o.thinlto.bc
> -fpropeller a.o -o lto/a.mir
>                 $(clang) -O2 -c -fthinlto-index=lto/b.o.thinlto.bc
> -fpropeller b.o -o lto/b.mir
>         propeller_link propeller/a.o propeller/b.o: lto/a.mir lto/b.mir
>                 # Propeller collects input Machine IR files,
>                 # spawn threads to generate object files parallelly.
>                 $(clang) -fpropeller-backend
> -fpropeller-prefix-replace='lto;propeller' lto/a.mir lto/b.mir
>         final_link exe: propeller/a.o propeller/b.o
>                 # GNU ld/gold/lld links object files.
>                 $(clang) $^ -o exe

There was an interesting talk last week at the LLVM performance
workshop: Global
Machine Outliner for ThinLTO <https://llvm.org/devmtg/2020-02-23/#kl> which
introduced a similar stage in ThinLTO (for another purpose though). I
believe they avoid the serialization of MIR by running the CodeGen twice
instead (once to collect the cross-module informations, and the second time
using these informations).
CC the author in case the slides are already available online.

> A .mir may be much large than an object file. So lto/a.mir may be
> actually an object file annotated with some information, or some lower
> level representation than a Machine IR (there should be a guarantee that
> the produced object file will keep the basic block structure unchanged
> => otherwise basic block profiling information will not be too useful).
> [1]: **Can we fix the bottleneck of full LTO** [1]?
> I wonder whether we have reached a "local maximum" of ThinLTO.
> If full LTO were nearly as fast as ThinLTO, how would we design a
> post-link optimization framework?
> Apparently, if full LTO did not have the scalability problem, we would
> not do so much work in the linker?

At lot of work went into ThinLTO because the scalability issue of LTO was
considered inherent to the design. It isn't clear what you're suggesting
here though?

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