When would that condition ever be false? s2v turns a pointer to a union into a pointer to its member, so isn't it basically just a cast? And i is just (o - base) modulo a cast, so that condition looks a lot like "base + o - base == o" to me.
Joseph C. Sible
On Sun, Feb 21, 2021 at 9:19 PM Yongheng Chen <
changochen1@gmail.com> wrote:
>
> Hi,
>
> In
https://github.com/lua/lua/blob/4e47f81188d37e29027158b76271d02a781242e2/ldebug.c#L645, the check for isinstack is
>
> `return (0 <= i && i < (ci->top - base) && s2v(base + i) == o);`
>
> However, I found that the default build (not modify anything in the makefile but only add `-g` to CFLAGS) of lua optimized away the final check (
> s2v(base + i) == o).
>
> The following is the asm of function varinfo (isinstack gets inlined in this function) in the binary built by GCC 8.3.0:
>
> .text:000000000000AEE5 add rdi, 10h
> .text:000000000000AEE9 mov rdx, o
> .text:000000000000AEEC sub rdx, rdi
> .text:000000000000AEEF js short loc_AF30
> .text:000000000000AEF1 cmp o, [ci+8]
> .text:000000000000AEF5 jnb short loc_AF30
> .text:000000000000AEF7 add rdi, rdx
> .text:000000000000AEFA cmp o, rdi
> .text:000000000000AEFD jnz short loc_AF30
> .text:000000000000AEFF mov rdi, [r8+18h] ; p
> .text:000000000000AF03 mov o, [ci+20h]
> .text:000000000000AF07 sar rdx, 4 ; reg
> .text:000000000000AF0B sub rsi, [rdi+40h]
> .text:000000000000AF0F sar rsi, 2
> .text:000000000000AF13 lea rcx, [rsp+18h+name] ; name
> .text:000000000000AF18 dec esi ; lastpc
> .text:000000000000AF1A call getobjname
>
> The instruction at 0xAEEF corresponds to i>=0, the instruction at 0x
> AEF5 refers to i < (ci->top - base). However, the third check is always true:
>
>
> .text:000000000000AEE9 mov rdx, o
> .text:000000000000AEEC sub rdx, rdi
> .text:000000000000AEF7 add rdi, rdx
> .text:000000000000AEFA cmp o, rdi
>
> Which means s2v(base + i) == o is constant-folded into a True. I also tried compiling with clang-7.0.1 and clang-9.0.1, which brought me the same result.
>
> I am not sure whether this is intended or will cause any problem.
>
> Best,
>
> Yongheng
