[Tarantool-patches] [PATCH luajit] ARM64: Fix assembly of HREFK.

[Sergey Bronnikov]

Hi, Sergey,

thanks for the patch! Please see my comments below.

Sergey

On 6/12/25 12:36, Sergey Kaplun wrote:
> From: Mike Pall <mike>
>
> Reported by caohongqing.
> Fix contributed by Peter Cawley.
>
> (cherry picked from commit 8fbd576fb9414a5fa70dfa6069733d3416a78269)
>
> `asm_hrefk()` uses the check for the offset for the corresponding node
> structure. However, the target load is performed from its inner `key`
> field with the offset 8. In the case of a huge table, it is possible
> that the offset of the node (4095 * 8) is less than 4096 * 8 and can be
> emitted via the corresponding instruction as an immediate offset, but
> the offset of the `key` field is not. This leads to the corresponding
> assertion failure in `emit_lso()`.

The issue [1] contains yet another fix in the same place [2]. We decided 
to backport the patch

separately. But please mention this in commit message.


1. https://github.com/LuaJIT/LuaJIT/issues/1026

2. 
https://github.com/LuaJIT/LuaJIT/commit/93ce12ee15abf28ef4cb24ae7e4b8a5b73d75c85


>
> This patch fixes this behaviour by the correct check.
>
> Sergey Kaplun:
> * added the description and the test for the problem
>
> Part of tarantool/tarantool#11278
> ---
>
> Related issues:
> *https://github.com/LuaJIT/LuaJIT/issues/1026
> *https://github.com/tarantool/tarantool/issues/11278
> Branch:https://github.com/tarantool/luajit/tree/skaplun/lj-1026-arm64-invalid-hrefk-offset-check
>
>   src/lj_asm_arm64.h                            |  2 +-
>   ...-arm64-invalid-hrefk-offset-check.test.lua | 48 +++++++++++++++++++
>   2 files changed, 49 insertions(+), 1 deletion(-)
>   create mode 100644 test/tarantool-tests/lj-1026-arm64-invalid-hrefk-offset-check.test.lua
>
> diff --git a/src/lj_asm_arm64.h b/src/lj_asm_arm64.h
> index 6c7b011f..a7f059a2 100644
> --- a/src/lj_asm_arm64.h
> +++ b/src/lj_asm_arm64.h
> @@ -885,7 +885,7 @@ static void asm_hrefk(ASMState *as, IRIns *ir)
>     IRIns *irkey = IR(kslot->op1);
>     int32_t ofs = (int32_t)(kslot->op2 * sizeof(Node));
>     int32_t kofs = ofs + (int32_t)offsetof(Node, key);
> -  int bigofs = !emit_checkofs(A64I_LDRx, ofs);
> +  int bigofs = !emit_checkofs(A64I_LDRx, kofs);
>     Reg dest = (ra_used(ir) || bigofs) ? ra_dest(as, ir, RSET_GPR) : RID_NONE;
>     Reg node = ra_alloc1(as, ir->op1, RSET_GPR);
>     Reg key, idx = node;
> diff --git a/test/tarantool-tests/lj-1026-arm64-invalid-hrefk-offset-check.test.lua b/test/tarantool-tests/lj-1026-arm64-invalid-hrefk-offset-check.test.lua
> new file mode 100644
> index 00000000..de243814
> --- /dev/null
> +++ b/test/tarantool-tests/lj-1026-arm64-invalid-hrefk-offset-check.test.lua
> @@ -0,0 +1,48 @@
> +local tap = require('tap')
> +
> +-- Test file to demonstrate LuaJIT misbehaviour when assembling
> +-- HREFK instruction on arm64 with the huge offset.
> +-- See also:https://github.com/LuaJIT/LuaJIT/issues/1026.
> +local test = tap.test('lj-1026-arm64-invalid-hrefk-offset-check'):skipcond({
> +  ['Test requires JIT enabled'] = not jit.status(),
It is an ARM-specific patch, should we add a condition for ARM here?
> +})
> +
> +test:plan(1)
> +
> +-- The assertion fails since in HREFK we are checking the offset
> +-- from the hslots of the table of the Node structure itself
s/Node/`Node`/
> +-- instead of its inner field `key` (with additional 8 bytes).
> +-- So to test this, we generate a big table with constant keys
> +-- and compile a trace for each HREFK possible.
> +
> +local big_tab = {}
> +-- The map of the characters to generate constant string keys.
> +-- The offset of the node should be 4096 * 8. It takes at least
> +-- 1365 keys to hit this value. The maximum possible slots in the
> +-- hash part is 2048, so to fill it with the maximum density (with
> +-- the way below), we need 45 * 45 = 2025 keys.
> +local chars = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRS'
> +for c1 inchars:gmatch('.') do
> +  for c2 inchars:gmatch('.') do
> +    big_tab[c1 .. c2] = 1
> +  end
> +end
> +
> +jit.opt.start('hotloop=1')
> +
> +-- Generate bunch of traces.
> +for c1 inchars:gmatch('.') do
> +  for c2 inchars:gmatch('.') do
> +    loadstring([=[
> +      local t = ...
> +      for i = 1, 4 do
> +        -- HREFK generation.
> +        t[ ']=] .. c1 .. c2 .. [=[' ] = i
> +      end
> +    ]=])(big_tab)
> +  end
> +end
> +
> +test:ok(true, 'no assertion failed')

I would replace testcase description to something like "emitted assembly 
is correct".

Feel free to ignore.

> +
> +test:done(true)
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