https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86582
--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #0)
> By modifying main to return i, I get in addition:
> ...
> .uleb128 0x7 # (DIE (0x77) DW_TAG_call_site_parameter)
> .uleb128 0x1 # DW_AT_location
> .byte 0x55 # DW_OP_reg5
> .uleb128 0x2 # DW_AT_call_value
> .byte 0x73 # DW_OP_breg3
> .sleb128 0
> ...
> and then gdb prints sizeof(a) as expected.
>
Which makes this test-case a duplicate of PR78685, and fkeep-vars-live would
help here.
> This seems to be cause DW_TAG_call_site_parameter is generated based on
> REG_CALL_ARG_LOCATION, and in the case without call_site_parameter we have:
> ...
> (call_insn 15 39 44 2 (set (reg:SI 0 ax)
> (call (mem:QI (symbol_ref:DI ("f1") [flags 0x3] <function_decl
> 0x7f7bc3fc2700 f1>) \
> [0 f1 S1 A8])
> (const_int 0 [0]))) "vla-1.c":19 722 {*call_value}
> (expr_list:REG_CALL_ARG_LOCATION (nil)
> (expr_list:REG_DEAD (reg:SI 5 di)
> (expr_list:REG_EH_REGION (const_int 0 [0])
> (nil))))
> (expr_list:SI (use (reg:SI 5 di))
> (nil)))
> ...
> and in the case with call_site_parameter we have:
> ...
> (call_insn 15 14 17 2 (set (reg:SI 0 ax)
> (call (mem:QI (symbol_ref:DI ("f1") [flags 0x3] <function_decl
> 0x7f701d65a700 f1>) \
> [0 f1 S1 A8])
> (const_int 0 [0]))) "vla-1.c":19 722 {*call_value}
> (expr_list:REG_CALL_ARG_LOCATION (expr_list:REG_DEP_TRUE (concat:SI
> (reg:SI 5 di)
> (reg:SI 3 bx [orig:89 i ] [89]))
> (nil))
> (expr_list:REG_DEAD (reg:SI 5 di)
> (expr_list:REG_EH_REGION (const_int 0 [0])
> (nil))))
> (expr_list:SI (use (reg:SI 5 di))
> (nil)))
> ...
>
> So, why doesn't var-tracking generate the REG_CALL_ARG_LOCATION that we need?
Because the register containing the arg is not guaranteed to be live across the
call:
...
.LVL7:
# DEBUG i => 0x5
#
/home/vries/gcc_versions/devel/src/gcc/testsuite/gcc.dg/guality/vla-1.c:18:3
.loc 1 18 3
movl $5, %edi
.LVL8:
# DEBUG i RESET
# DEBUG i => di
#
/home/vries/gcc_versions/devel/src/gcc/testsuite/gcc.dg/guality/vla-1.c:19:3
.loc 1 19 3
#
/home/vries/gcc_versions/devel/src/gcc/testsuite/gcc.dg/guality/vla-1.c:19:7
.loc 1 19 7 is_stmt 0
call f1
.LVL9:
# DEBUG i RESET
...
If we modify main to return i, we have instead:
...
.LVL7:
# DEBUG i => 0x5
#
/home/vries/gcc_versions/devel/src/gcc/testsuite/gcc.dg/guality/vla-1.c:18:3
.loc 1 18 3
movl $5, %ebx
.LVL8:
# DEBUG i RESET
# DEBUG i => bx
#
/home/vries/gcc_versions/devel/src/gcc/testsuite/gcc.dg/guality/vla-1.c:19:3
.loc 1 19 3
#
/home/vries/gcc_versions/devel/src/gcc/testsuite/gcc.dg/guality/vla-1.c:19:7
.loc 1 19 7 is_stmt 0
movl %ebx, %edi
call f1
.LVL9:
...
