On Sat, May 25, 2024 at 09:13:12AM -0300, Alexandre Oliva wrote:
> Some of the rs6000 call patterns, on some ABIs, issue multiple opcodes
> out of a single call insn, but the call (bl) or jump (b) is not always
> the last opcode in the sequence.
> This does not seem to be a problem for exception handling tables, but
> the return_pc attribute in the call graph output in dwarf2+ debug
> information, that takes the address of a label output right after the
> call, does not match the value of the link register even for non-tail
> calls. E.g., with ABI_AIX or ABI_ELFv2, such code as:
>
> foo ();
>
> outputs:
>
> bl foo
> nop
> LVL#:
> [...]
> .8byte .LVL# # DW_AT_call_return_pc
>
> but debug info consumers may rely on the return_pc address, and draw
> incorrect conclusions from its off-by-4 value.
>
> This patch uses the infrastructure for targets to add an offset to the
> label issued after the call_insn to set the call_return_pc attribute,
> on rs6000, to account for opcodes issued after actual call opcode as
> part of call insns output patterns.
> for gcc/ChangeLog
>
> * config/rs6000/rs6000.cc (TARGET_CALL_OFFSET_RETURN_LABEL):
> Override.
Please don't (incorrectly!) line-wrap changelogs. Lines are 80
characters wide, not 60 or 72 or whatever. 80. Indents are tabs that
take 8 columns.
> +/* Return the offset to be added to the label output after CALL_INSN
> + to compute the address to be placed in DW_AT_call_return_pc. */
> +
> +static int
> +rs6000_call_offset_return_label (rtx_insn *call_insn)
> +{
> + /* All rs6000 CALL_INSN output patterns start with a b or bl, always
This isn't true. There is a crlogical insn before the bcl for sysv for
example.
> + a 4-byte instruction, but some output patterns issue other
> + opcodes afterwards. The return label is issued after the entire
> + call insn, including any such post-call opcodes. Instead of
> + figuring out which cases need adjustments, we compute the offset
> + back to the address of the call opcode proper, then add the
> + constant 4 bytes, to get the address after that opcode. */
> + return 4 - get_attr_length (call_insn);
Please explain this magic, too -- in code preferably (so with a ? :
maybe, but don't try to "optimise" that expression, let the compiler
do that, it is much better at it anyway :-) )
> +}
Is that correct for all ABIs we support? Even if so, it needs a lot
more documentation than this.
Segher
