[llvm-dev] [RFC] Propeller: A frame work for Post Link Optimizations

Tue Oct 8 13:19:04 PDT 2019

Some more information about the relaxation pass whose effectiveness
and convergence guarantees were listed as a concern:

TLDR; Our relaxation pass is similar to what LLVM’s MCAssembler does
but with a caveat for efficiency.  Our experimental results show it is
efficient and convergence is guaranteed.

Our relaxation pass is very similar to what MCAssembler does as it
needs to solve the same problem, which is laying out basic blocks and
optimizing branch instruction sizes based on jump offset.  The optimal
algorithm to do this is quadratic and it involves computating of
section offsets multiple times. Our relaxation pass has a caveat for
efficiency. Our relaxation pass recomputes section offsets only after
all the sections have been processed. This is repeated  until
convergence, whereas, the MCAssembler re-computes section offsets
every time a section is modified. Our pass works well in the linker as
it minimizes the section offset recomputation, which is more expensive
to do in the linker (whole program) than doing it in the assembler
(per function).

We shrink jump instructions aggressively and then fix the wrongly
shrunk branches by running a pass that grows them back.  We guarantee
convergence because we always shrink in the first round and always
grow in the second.

Data shows that our relaxation is efficient. For example, for clang,
the aggressive shrinking affects ~995000 branches in 6 iterations and
only 4 branches had to be grown back.

On Mon, Oct 7, 2019 at 11:17 AM Sriraman Tallam <tmsriram at google.com> wrote:
>
> The next concern we want to address is about C++ Exceptions which was
> considered a major omission:
>
> TLDR;  This can be implemented in propeller easily. It was considered
> low priority because exception handling code is assumed in general not
> performance critical .
>
> Currently, we do not create basic block sections for any block that is
> involved in exception handling.   There is nothing fundamentally
> difficult about exception handling basic blocks. The exception table
> uses address offsets which currently does not use relocations, and we
> must modify it to use relocations in order to enable sections for
> exception handling basic blocks.
>
> We will re-prioritize and send out a patch to handle exception basic blocks.
>
> Thanks
> Sri
>
> On Mon, Oct 7, 2019 at 11:15 AM Sriraman Tallam <tmsriram at google.com> wrote:
> >
> > We would also like to clarify on the misconceptions around CFI Instructions:
> >
> > There are two things that need to be clarified here:
> >
> > 1) Extra CFI FDE entries for basic blocks does not mean more dynamic
> > instructions are executed. In fact, they do not increase at all.  Krys
> > talked about this earlier.
> > 2) We do deduplication of common static CFI instructions in the FDE
> > and move it to the CIE .  Hence, moving to a new basic block does not
> > mean a completely new set of CFI instructions is executed.
> >
> > On Wed, Oct 2, 2019 at 7:24 PM Sriraman Tallam <tmsriram at google.com> wrote:
> > >
> > > Maks and team, thanks for the detailed feedback and we will address all of your
> > > concerns.  Let’s begin with CFI and DebugInfo first since this is already
> > > being discussed.
> > >
> > > TLDR; clang is pathological and the actual CFI bloat will go down from 7x to
> > > 2x.
> > >
> > > Let us present the CFI Bloats for each benchmark with the default option, which
> > > is creating basic block sections only for functions with samples.   For clang,
> > > it is 7x and *not 17x* (the 17 x number is for all sections), and for the
> > > large benchmarks it is less than 30% or 1.3x. For large benchmarks, Storage is
> > > the highest, going from 18M to 23M, ~30%. Clang is almost pathological here.
> > > This is because 10% of all functions in clang have samples (touched by the
> > > profile information), and all of these functions get full basic block sections.
> > > Whereas, for the large benchmarks, only 0.5% of functions have samples.  Now,
> > > for clang, 10% of functions that have samples also happen to contain 35% of all
> > > basic blocks. This means, we are creating sections for 35% of all basic blocks
> > > and the CFI bloats are clearly showing.
> > >
> > > Now, the data for clang also shows that only 7% of the basic blocks have
> > > samples. We are working on restricting basic block sections to only those basic
> > > blocks that have samples. The rest of the basic blocks (cold) in that function
> > > would share the same section.  With this, we would reduce the bloat of CFI
> > > from 7x to 2x. This is not hard to do and we will follow up with this patch.
> > >
> > > Also for object size bloats with regards to eh_frame, the reasoning is similar.
> > >  Restricting the section creation to only basic blocks that have profiles will
> > > reduce this a lot more.
> > >
> > > Importantly,  if CFI support were available for discontiguous ranges we wouldn't
> > > have to duplicate CFI FDEs and the bloats would be near minimal.
> > >
> > > BOLT parses CFI and DWARF and generates compact information by rewriting it.
> > > Whereas, Propeller uses lld which uses relocations and sections to fixup but
> > > does not rewrite it.  This is by design and that lld is not DWARF and CFI
> > > aware. We designed basic block sections just like function sections.  The
> > > compiler produces a bunch of sections and relocations. The linker patches the
> > > relocations and the debug info and CFI are right, that's it.  For CFI, since
> > > there is no support for discontiguous ranges we have to duplicate and dedup
> > > FDEs only for blocks with sections. We are asking that CFI support
> > > discontiguous ranges and this would look even simpler.  Alternately, if lld
> > > were made DWARF and CFI aware we could rewrite it compactly like BOLT.
> > > These would help with object size bloats and binary size bloats.
> > >
> > >
> > >
> > >
> > >
> > > On Wed, Oct 2, 2019 at 1:59 PM James Y Knight via llvm-dev
> > > <llvm-dev at lists.llvm.org> wrote:
> > > >
> > > > I'm a bit confused by this subthread -- doesn't BOLT have the exact same CFI bloat issue? From my cursory reading of the propellor doc, the CFI duplication is _necessary_ to represent discontiguous functions, not anything particular to the way Propellor happens to generate those discontiguous functions.
> > > >
> > > > And emitting discontiguous functions is a fundamental goal of this, right?
> > > >
> > > > On Wed, Oct 2, 2019 at 4:25 PM Maksim Panchenko via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> > > >>
> > > >> Thanks for clarifying. This means once you move to the next basic block (or any other basic
> > > >>
> > > >> block in the function) you have to execute an entirely new set of CFI instructions
> > > >>
> > > >> except for the common CIE part. While indeed this is not as bad, on average, the overall
> > > >>
> > > >> active memory footprint will increase.
> > > >>
> > > >>
> > > >>
> > > >> Creating one FDE per basic block means that .eh_frame_hdr, an allocatable section,
> > > >>
> > > >> will be bloated too. This will increase the FDE lookup time. I don’t see .eh_frame_hdr
> > > >>
> > > >> being mentioned in the proposal.
> > > >>
> > > >>
> > > >>
> > > >> Maksim
> > > >>
> > > >>
> > > >>
> > > >> On 10/2/19, 12:20 PM, "Krzysztof Pszeniczny" <kpszeniczny at google.com> wrote:
> > > >>
> > > >>
> > > >>
> > > >>
> > > >>
> > > >>
> > > >>
> > > >> On Wed, Oct 2, 2019 at 8:41 PM Maksim Panchenko via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> > > >>
> > > >> *Pessimization/overhead for stack unwinding used by system-wide profilers and
> > > >> for exception handling*
> > > >>
> > > >> Larger CFI programs put an extra burden on unwinding at runtime as more CFI
> > > >> (and thus native) instructions have to be executed. This will cause more
> > > >> overhead for any profiler that records stack traces, and, as you correctly note
> > > >> in the proposal, for any program that heavily uses exceptions.
> > > >>
> > > >>
> > > >>
> > > >> The number of CFI instructions that have to be executed when unwinding any given stack stays the same. The CFI instructions for a function have to be duplicated in every basic block section, but when performing unwinding only one such a set is executed -- the copy for the current basic block. However, this copy contains precisely the same CFI instructions as the ones that would have to be executed if there were no basic block sections.
> > > >>
> > > >>
> > > >>
> > > >> --
> > > >>
> > > >> Krzysztof Pszeniczny
> > > >>
> > > >> _______________________________________________
> > > >> LLVM Developers mailing list
> > > >> llvm-dev at lists.llvm.org
> > > >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > > >
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