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From: Igor Munkin via Tarantool-patches <tarantool-patches@dev.tarantool.org>
To: Sergey Ostanevich <sergos@tarantool.org>
Cc: tarantool-patches@dev.tarantool.org
Subject: Re: [Tarantool-patches] [PATH luajit] GC64: fix 64-bit constant fusion
Date: Mon, 5 Jul 2021 00:06:47 +0300	[thread overview]
Message-ID: <20210704210647.GA6106@tarantool.org> (raw)
In-Reply-To: <804A99A3-6D0C-4DA9-A939-26FFED0EC823@tarantool.org>


Thanks for the patch! Something went wrong with formatting the patch I
guess, so I'll share you the recipe how I send the backported patches:
  1. cherry-pick the original patch from the upstream
  2. format-patch the backported commit
  3. Add "From" header with the original author of the changes at the
     very beginning of the formatted patch
  4. Adjust the commit message according to our backporting procedure

BTW, I've found neither the branch with the patch in tarantool/luajit
nor the branch with the green CI in tarantool/tarantool...

On 28.05.21, Sergey Ostanevich wrote:
> Author: Mike Pall <mike>
> Date:   Mon Aug 28 10:43:37 2017 +0200
>     x64/LJ_GC64: Fix fallback case of asm_fuseloadk64().
>     Contributed by Peter Cawley.
>     (cherry picked from commit 6b0824852677cc12570c20a3211fbfe0e4f0ce14)
>     Code generation under LJ_GC64 missed an update to the mcode area after

Minor: s/mcode area/mcode area boundaries/. Feel free to ignore.

>     a 64bit constant encoding. This lead to a corruption to the constant

Typo: s/corruption to/corruption of/.

>     later on.
>     The problem is rather rare, since there should be big enough (4GiB)
>     distance from the currently allocated mcode to the dispatch pointer.

Minor: Sorry for being pedantic, but this is not such a trivial bug, so
I've described semantics around this a bit. As for me it's worth to
mention four possible encodings of 64-bit constant on the trace.
  0. If the address of the constant fits into 32-bit one, then encode it
     as a 32-bit displacement (the only option for non-GC64 mode).
  1. If the offset of the constant slot from the dispatch table (pinned
     to r14 that is not changed while trace execution) fits into 32-bit,
     then encode this as a 32-bit displacement relative to r14.
  2. If the offset of the constant slot from the mcode (i.e. rip) fits
     into 32-bit, then encode this as a 32-bit displacement relative to
     rip (considering long mode specifics and RID_RIP hack).
  3. If none of the conditions above are valid, compiler materializes
     this 64-bit constant right at the trace bottom and encodes this the
     same way it does for the previous case.

And now goes your part regarding 4Gb and rarity of this case and the
problem is much clearer to the reader. Please adjust this part using my
description above.

>     This lead to a number of flaky tests, trackers are addressed.
>     Sergey Ostanevich:
>     * added the description and the test for the problem
>     Closes: #4095, #4199, #4614

At first, please move split this line into the separate ones with a
single issue per line. Also don't use ':' in GitHub tags, please: it
just doesn't respect our commit message style. Unfortunately, this
commit tags no issue, since it is pushed to tarantool/luajit repo, but
the issues relate to tarantool/tarantool. Hence, you need to explicitly
mention the latter repo.  And last (but not least), we agreed with
Sergey to use "Resolves" instead of "Closes" in tarantool/luajit repo
and use "Closes" in the corresponding patches bumping LuaJIT submodule.
As for these patches, I'd rather use "Fixes", since you have checked
that the failures related to the mentioned issues are gone. Considering
everything above, the line above transforms into the following:

| Fixes tarantool/tarantool#4095
| Fixes tarantool/tarantool#4199
| Fixes tarantool/tarantool#4614

>     Signed-off-by: Sergey Ostanevich <sergos@tarantool.org>
> diff --git a/src/lj_asm_x86.h b/src/lj_asm_x86.h
> index 767bf6f3..2850aea9 100644
> --- a/src/lj_asm_x86.h
> +++ b/src/lj_asm_x86.h
> @@ -387,6 +387,7 @@ static Reg asm_fuseloadk64(ASMState *as, IRIns *ir)
>        ir->i = (int32_t)(as->mctop - as->mcbot);
>        as->mcbot += 8;
>        as->mclim = as->mcbot + MCLIM_REDZONE;
> +      lj_mcode_commitbot(as->J, as->mcbot);
>      }
>      as->mrm.ofs = (int32_t)mcpofs(as, as->mctop - ir->i);
>      as->mrm.base = RID_RIP;
> diff --git a/test/tarantool-tests/gh-4199-gc64-flaky.test.lua b/test/tarantool-tests/gh-4199-gc64-flaky.test.lua
> new file mode 100644
> index 00000000..3ac30427
> --- /dev/null
> +++ b/test/tarantool-tests/gh-4199-gc64-flaky.test.lua
> @@ -0,0 +1,63 @@

Regarding test code style: I don't want to point each place you have
violated it, so I just refer the document[1] that we try to follow except
the one rule: we use 2 spaces for indentation to get tests closer to
LuaJIT sources. Hence, please consider that I've "implicitly" commented
every spot below, where you don't follow our code style.

> +-- the test is GC64 only
> +local ffi=require('ffi')
> +require('utils').skipcond(not ffi.abi('gc64'), 'test is GC64 only')
> +
> +local tap = require("tap")
> +local test = tap.test("gh-4199-gc64-flaky")
> +test:plan(1)
> +
> +-- first - we have to make a gap from current JIT infra to next
> +-- available mappable memory
> +-- most efficient is to grab it per-page
> +
> +
> +ffi.cdef('void * mmap(void *start, size_t length, int prot , int flags, int fd, long offset);')
> +ffi.cdef('long getpagesize();')

Minor: It's better use <ffi.cdef> once but with multiline declarations
instead of several <ffi.cdef> calls.

> +
> +local pagesize = tonumber(ffi.C.getpagesize())
> +local blob = {}
> +for i=1, 4e9/pagesize do
> +        blob[i] = ffi.C.mmap(ffi.cast('void*',0), pagesize, 0, 0x22, 0, 0)

0x22, 4e9 -- these are magic numbers for me. Please, create a variable
with the descriptive name (and even comments, I guess) for each of them,
to ease the further maintenance.

> +        assert(blob[i] ~= 0)
> +end
> +
> +-- try to chomp all memory in currently allocated gc space
> +collectgarbage('stop')

Since you decided to stop GC, you can do it right at the very beginning,
so you don't need to anchor all mmaped memory above.

> +local dummy={'a'}
> +for i=2,30 do
> +        dummy[i] = dummy[i - 1] .. dummy[i - 1]
> +end

Why do you need this loop?

> +
> +-- generate a bunch of functions and keep them stored to trigger wrong constant placement
> +
> +local s={}

Again, since GC is stopped, there is no need to anchor all the functions
generated below.

> +local pass = true
> +
> +jit.opt.start('hotloop=1’)
> +for n=1,100 do
> +        local src='function f'.. n .. [[(x,y,z,f,g,h,j,k,r,c,d)
> +                local a={}
> +                for i=1,1e6 do
> +                        a[i] = x + y + z + f + g + h + j + k + r + c + d
> +                        if (x > 0) then a[i] = a[i] + 1.1 end
> +                        if (c > 0) then a[i] = a[i] + 2.2 end
> +                        if (z > 0) then a[i] = a[i] + 3.3 end
> +                        if (f > 0) then a[i] = a[i] + 4.4 end
> +                        x=x+r
> +                        y=y-c
> +                        z=z+d
> +                end
> +                return a[1]
> +        end
> +        return f]] .. n ..'(...)'
> +
> +        s[n] = assert(load(src))
> +        local res1 = s[n](1,2,3,4,5,6,7,8,9,10,11)
> +        local res2 = s[n](1,2,3,4,5,6,7,8,9,10,11)

This was not obvious to me, but I've finally got it: you compare the
result yielded by interpreter with the one yielded by the trace. At
first, you don't need to run all 1e6 iterations: you have set 'hotloop'
to 1, so you need only 3 (!) iterations to successfully compile the
loop. The second call will use the compiled trace for the first loop
iteration, but also will try to compile the function itself. AFAIU, the
constant being materialized on the trace is the <a> table address,
right? At least I see no other option, so please mention in the comment
which constant is fused and leads to the failure if the patch is
missing. Finally, I don't get, why do you need 100 iterations for
getting the misbehaviour.

Please, add everything I dumped below as the corresponding comments for
this part.

> +        if (res1 ~= res2) then
> +                pass = false

Why don't you add the assertion about constant fusion right here?

> +                break
> +        end
> +end
> +
> +test:ok(pass, 'wrong IR constant fuse')

[1]: https://www.tarantool.io/en/doc/latest/dev_guide/lua_style_guide/

Best regards,

  reply	other threads:[~2021-07-04 21:30 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-28 12:06 Sergey Ostanevich via Tarantool-patches
2021-07-04 21:06 ` Igor Munkin via Tarantool-patches [this message]
2022-02-16 15:44   ` Sergey Kaplun via Tarantool-patches
2022-06-21 12:11     ` sergos via Tarantool-patches
2022-06-22 13:32       ` Sergey Kaplun via Tarantool-patches
2022-06-29  8:04         ` Igor Munkin via Tarantool-patches
2022-06-30 12:10 ` Igor Munkin via Tarantool-patches

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