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

[sergos]

Hi! 

I see the test is guess-based in term that number of traces should
be enough to trigger the situation. Is there any confirmation it
will happen at particular traces count?

Otherwise LGTM.

Sergos

> On 27 Oct 2021, at 16:02, Sergey Kaplun <skaplun at tarantool.org> wrote:
> 
> From: Mike Pall <mike>
> 
> Reported by Jason Teplitz.
> 
> (cherry picked from commit 06cd9fce7df440323647174f1ca4a01281ec8acd)
> 
> LDR instruction with register offset [1] has a restriction, that the
> destination register must be different from the register used for offset
> base. HREFK IR emits LDR instruction for a load of node key to a
> register. Base register to offset contains a node address. The register
> holding the node address is not excluded from a allow set, when loading
> the key value to a new register, and may be chosen by the register
> allocator as a destination for the key value, which violates the
> aforementioned restriction.
> 
> This patch excludes the node register from the allowing set in the
> allocation of register for a key value.
> 
> Sergey Kaplun:
> * added the description and the test for the problem
> 
> [1]: https://developer.arm.com/documentation/dui0489/c/arm-and-thumb-instructions/memory-access-instructions/ldr-and-str--register-offset-
> 
> Part of tarantool/tarantool#6548
> ---
> 
> Branch: https://github.com/tarantool/luajit/tree/skaplun/lj-357-arm64-hrefk
> Tarantool branch: https://github.com/tarantool/tarantool/tree/skaplun/lj-357-arm64-hrefk
> LuaJIT issue: https://github.com/LuaJIT/LuaJIT/issues/357
> Issue: https://github.com/tarantool/tarantool/issues/6548
> 
> src/lj_asm_arm64.h                            | 11 ++++----
> .../lj-357-arm64-hrefk.test.lua               | 27 +++++++++++++++++++
> 2 files changed, 32 insertions(+), 6 deletions(-)
> create mode 100644 test/tarantool-tests/lj-357-arm64-hrefk.test.lua
> 
> diff --git a/src/lj_asm_arm64.h b/src/lj_asm_arm64.h
> index cc8c0c9d..da0ee4bb 100644
> --- a/src/lj_asm_arm64.h
> +++ b/src/lj_asm_arm64.h
> @@ -869,14 +869,12 @@ static void asm_hrefk(ASMState *as, IRIns *ir)
>   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);
> -  RegSet allow = RSET_GPR;
>   Reg dest = (ra_used(ir) || bigofs) ? ra_dest(as, ir, RSET_GPR) : RID_NONE;
> -  Reg node = ra_alloc1(as, ir->op1, allow);
> -  Reg key = ra_scratch(as, rset_clear(allow, node));
> -  Reg idx = node;
> +  Reg node = ra_alloc1(as, ir->op1, RSET_GPR);
> +  Reg key, idx = node;
> +  RegSet allow = rset_exclude(RSET_GPR, node);
>   uint64_t k;
>   lua_assert(ofs % sizeof(Node) == 0);
> -  rset_clear(allow, key);
>   if (bigofs) {
>     idx = dest;
>     rset_clear(allow, dest);
> @@ -892,7 +890,8 @@ static void asm_hrefk(ASMState *as, IRIns *ir)
>   } else {
>     k = ((uint64_t)irt_toitype(irkey->t) << 47) | (uint64_t)ir_kgc(irkey);
>   }
> -  emit_nm(as, A64I_CMPx, key, ra_allock(as, k, allow));
> +  key = ra_scratch(as, allow);
> +  emit_nm(as, A64I_CMPx, key, ra_allock(as, k, rset_exclude(allow, key)));
>   emit_lso(as, A64I_LDRx, key, idx, kofs);
>   if (bigofs)
>     emit_opk(as, A64I_ADDx, dest, node, ofs, RSET_GPR);
> diff --git a/test/tarantool-tests/lj-357-arm64-hrefk.test.lua b/test/tarantool-tests/lj-357-arm64-hrefk.test.lua
> new file mode 100644
> index 00000000..200d29f0
> --- /dev/null
> +++ b/test/tarantool-tests/lj-357-arm64-hrefk.test.lua
> @@ -0,0 +1,27 @@
> +local tap = require('tap')
> +
> +-- Test file to demonstrate the incorrect JIT behaviour for HREFK
> +-- IR compilation on arm64.
> +-- See also https://github.com/LuaJIT/LuaJIT/issues/357.
> +local test = tap.test('lj-357-arm64-hrefk')
> +test:plan(2)
> +
> +jit.opt.start('hotloop=1', 'hotexit=1')
> +
> +local t = {hrefk = 0}
> +
> +-- XXX: Need to generate a bunch of side traces (starts a new one
> +-- when the hmask is changed) to wait, when the register allocator
> +-- chooses the same register as a base register for offset and
> +-- destination in LDR instruction.
> +local START = 2e3
> +local STOP = 1
> +for i = START, STOP, -1 do
> +  t.hrefk = t.hrefk - 1
> +  t[t.hrefk] = i
> +end
> +
> +test:is(t.hrefk, -START)
> +test:is(t[t.hrefk], STOP)
> +
> +os.exit(test:check() and 0 or 1)
> -- 
> 2.31.0
>