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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