Re: [Valgrind-developers] [LLVMdev] [GSoC 2014] Using LLVM as a code-generation backend for Valgrind

[Yan <ya...@ya...>]

For (3), would something like making all statements conditional (like
LoadG, StoreG, and Exit are) do, or are we talking about something more
complex?


On Wed, Feb 26, 2014 at 7:16 AM, Julian Seward <js...@ac...> wrote:

>
> On 02/26/2014 12:23 PM, Kirill Batuzov wrote:
>
> I tend to agree with Kirill.  It would be great to make Valgrind/Memcheck
> faster, and there are certainly ways to do that, but using LLVM is not
> one of them.
>
> > Second, in DBT you translate code in small portions like basic blocks,
> > or extended basic blocks. They have very simple structure. There is no
> > loops, there is no redundancy from translation high level language to
> > low level. There is nothing good sophisticated optimizations can do
> > better then very simple ones.
>
> Yes.  One of the problems of the "Let's use LLVM and it'll all go much
> faster" concept is that it lacks a careful analysis of what makes Valgrind
> (and QEMU, probably) run slowly in the first place.
>
> As Kirill says, the short blocks of code that V generates make it
> impossible for LLVM to do sophisticated loop optimisations etc.
> Given what Valgrind's JIT has to work with -- straight line pieces
> of code -- it generally does a not-bad job of instruction selection
> and register allocation, and I wouldn't expect that substituting LLVM's
> implementation thereof would make much of a difference.
>
> What would make Valgrind faster is
>
> (1) improve the caching of guest registers in host registers across
>     basic block boundaries.  Currently all guest registers cached in
>     host registers are flushed back into memory at block boundaries,
>     and no host register holds any live value across the boundary.
>     This is simple but very suboptimal, creating large amounts of
>     memory traffic.
>
> (2) improve the way that the guest program counter is represented.
>     Currently it is updated before every memory access, so that if an
>     unwind is required, it is possible.  But this again causes lots of
>     excess memory traffic.  This is closely related to (1).
>
> (3) add some level of control-flow if-then-else support to the IR, so
>     that the fast-case paths for the memcheck helper functions
>     (helperc_LOADV64le etc) can be generated inline.
>
> (4) Redesign Memcheck's shadow memory implementation to use a 1 level
>     map rather than 2 levels as at present.  Or something more
>     TLB-like.
>
> I suspect that the combination of (1) and (2) causes processor write
> buffers to fill up and start stalling, although I don't have numbers
> to prove that.  What _is_ very obvious from profiling Memcheck using
> Cachegrind is that the generated code contains much higher proportion
> of memory references than "normal integer code".  And in particular
> it contains perhaps 4 times as many stores as "normal integer code".
> Which can't be a good thing.
>
> (3) is a big exercise -- much work -- but potentially very beneficial.
> (4) is also important if only because we need a multithreaded
> implementation of Memcheck.  (1) and (2) are smaller projects and would
> constitute a refinement of the existing code generation framework.
>
> > In conclusion I second what have already been said: this project sounds
> > like fun to do, but do not expect much practical results from it.
>
> The above projects (1) .. (4) would also be fun :-) and might generate more
> immediate speedups for Valgrind.
>
> J
>
>
>
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