Re: [Tarantool-patches] [PATCH luajit] ARM64: Fix LDP/STP fusing for unaligned accesses.

[Sergey Bronnikov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

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Thanks! LGTM

On 6/30/25 10:26, Sergey Kaplun wrote:
> Hi, Sergey!
> Thanks for the review!
> Fixed your comments and force-pushed the branch.
>
> On 27.06.25, Sergey Bronnikov wrote:
>> Hi, Sergey,
>>
>> thanks for the patch! LGTM with a minor two comments below.
>>
>> Sergey
>>
>> On 6/26/25 18:12, Sergey Kaplun wrote:
>>> From: Mike Pall <mike>
>>>
>>> Thanks to Peter Cawley.
>>>
>>> (cherry picked from commit 0fa2f1cbcf023ad0549f1428809e506fa2c78552)
>>>
>>> The arm64 emitting of load/store operation works incorrectly in the
>>> case when at least one offset of load/store to be fused into ldp/stp is
>>> misaligned. In this case this misaligning is ignored, and instructions
>>> are fused, which leads to loading/storing from/to at least one incorrect
>>> address.
>>>
>>> For example, the following instructions:
>>> | stur  w0, [x1, #17]
>>> | stur  w0, [x1, #21]
>>>
>>> May be fused to the following:
>>> | stp   w0, w0, [x1, #16]
>>>
>>> This patch prevents fusion in this case by testing the alignment with
>>> the help of bitwise ROR by the alignment value. In case of misaligned
>>> offset, the value overflows the 7-bit length mask in the check.
>>>
>>> The negative immediate (7-bit width including sign bit) is limited by
>>> the corresponding addition of `64 << sc` (it is harmless in the case of
>>> positive values).
>>>
>>> Sergey Kaplun:
>>> * added the description and the test for the problem
>>>
>>> Part of tarantool/tarantool#11278
>>> ---
>>>
>>> Related issues:
>>> *https://github.com/LuaJIT/LuaJIT/issues/1056
>>> *https://github.com/tarantool/tarantool/issues/11278
>>> Branch:https://github.com/tarantool/luajit/tree/skaplun/lj-1056-arm64-ldp-sdp-misaligned-fusing
>>>
>>>    src/lj_emit_arm64.h                           |  2 +-
>>>    ...6-arm64-ldp-sdp-misaligned-fusing.test.lua | 98 +++++++++++++++++++
>>>    2 files changed, 99 insertions(+), 1 deletion(-)
>>>    create mode 100644 test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
>>>
>>> diff --git a/src/lj_emit_arm64.h b/src/lj_emit_arm64.h
>>> index 30cd3505..5c1bc372 100644
>>> --- a/src/lj_emit_arm64.h
>>> +++ b/src/lj_emit_arm64.h
> <snipped>
>
>>> diff --git a/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua b/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
>>> new file mode 100644
>>> index 00000000..5d03097e
>>> --- /dev/null
>>> +++ b/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
>>> @@ -0,0 +1,98 @@
>>> +local tap = require('tap')
>>> +local ffi = require('ffi')
>>> +
>>> +-- This test demonstrates LuaJIT's incorrect emitting of LDP/STP
>>> +-- instructions from LDUR/STUR instructions with misaligned offset
>>> +-- on arm64.
>>> +-- Seealsohttps://github.com/LuaJIT/LuaJIT/issue/1056.
>> s/issue/issues/
> Fixed, thanks!
>
> ===================================================================
> diff --git a/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua b/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
> index 815da15d..5ff040e7 100644
> --- a/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
> +++ b/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
> @@ -4,7 +4,7 @@ local ffi = require('ffi')
>   -- This test demonstrates LuaJIT's incorrect emitting of LDP/STP
>   -- instructions from LDUR/STUR instructions with misaligned offset
>   -- on arm64.
> --- See alsohttps://github.com/LuaJIT/LuaJIT/issue/1056.
> +-- See alsohttps://github.com/LuaJIT/LuaJIT/issues/1056.
>   local test = tap.test('lj-1056-arm64-ldp-sdp-misaligned-fusing'):skipcond({
>     ['Test requires JIT enabled'] = not jit.status(),
>   })
> ===================================================================
>
>>> +local test = tap.test('lj-1056-arm64-ldp-sdp-misaligned-fusing'):skipcond({
>>> +  ['Test requires JIT enabled'] = not jit.status(),
>>> +})
>>> +
> <snipped>
>
>>> +
>>> +test:is(resl, 0x4030201, 'pair of misaligned loads, left')
>>> +test:is(resr, 0x8070605, 'pair of misaligned loads, right')
>> What does mean these magic numbers? Please add a comment or
>>
>> use a variable with self-explained name. Here and below.
> Added the comments nearby, see the corresponding patch below:
>
> ===================================================================
> diff --git a/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua b/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
> index 5d03097e..815da15d 100644
> --- a/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
> +++ b/test/tarantool-tests/lj-1056-arm64-ldp-sdp-misaligned-fusing.test.lua
> @@ -81,6 +81,8 @@ for _ = 1, N_ITERATIONS do
>     resr = ffi.cast('int32_t *', ptr + 5)[0]
>   end
>   
> +-- Values are resulted from the `init_buf()` function with the
> +-- corresponding offset.
>   test:is(resl, 0x4030201, 'pair of misaligned loads, left')
>   test:is(resr, 0x8070605, 'pair of misaligned loads, right')
>   
> @@ -92,6 +94,8 @@ for _ = 1, N_ITERATIONS do
>     resl = ffi.cast('int32_t *', ptr)[0]
>   end
>   
> +-- Values are resulted from the `init_buf()` function with the
> +-- corresponding offset.
>   test:is(resl, 0x3020100, 'aligned / misaligned load, aligned')
>   test:is(resr, 0x8070605, 'aligned / misaligned load, misaligned')
>   
> ===================================================================
>
> <snipped>
>

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