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

Wed Oct 2 19:24:05 PDT 2019

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
>>
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>
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