What is the precise definition of NOTE_INSN_FUNCTION_BEG?

[Jeff Law]

On 1/4/19 9:03 AM, Matthew Malcomson wrote:
> Hi there,
> 
> I'm trying to figure out precisely what NOTE_INSN_FUNCTION_BEG means and 
> hoping someone here knows.
It doesn't mean very much anymore.   I believe it was used to
distinguish between stuff like copying incoming arguments into pseudos
and real user code.

However, in a world with instruction scheduling and other code motion it
just doesn't have much use because it's so imprecise, particularly as we
get deeper into the RTL pipeline.

> 
> The gcc internal documentation says it "Appears at the start of the 
> function body, after the function prologue." but this isn't very 
> precise, and it seems to not distinguish between the undocumented 
> NOTE_INSN_PROLOGUE_END and the NOTE_INSN_FUNCTION_BEG.
> 
> A comment in dwarf2out.c shows it's used as a marker between boilerplate 
> and code corresponding to things "the user wrote".
> This assumption is broken by things like -fstack-protector-strong (as 
> bugzilla 88432 shows).
That was the intent, but I think it's a concept that's fundamentally
flawed through much of the RTL pipeline these days.

> 
> alias.c seems to use it to assume that argument registers have their 
> original values before it (see the use of the "copying_arguments" 
> variable and find_base_value function).
> I'm not yet certain of this, but if that is the assumption I think this 
> is also not observed.
> 
> The test file below compiles to something breaking that assumption when 
> compiled with `gcc -fsanitize=address -S test.c -o test.s -g3 
> -fdump-rtl-final -O0` on aarch64.
> 
> #include <string.h>
> #include <stdio.h>
> int main (int argc, char *argv[]) {
>  Â Â Â Â Â Â Â  char buf[64];
>  Â Â Â Â Â Â Â  return !strcpy (buf, strrchr (argv[0], '/'));
> }
> 
> 
> Is there any tight meaning to NOTE_INSN_FUNCTION_BEG?
> 
> I'm wondering whether it should be split into one note to determine for 
> the debugging use and another for the use in alias.c.
Well, the question I'd ask is precisely what are you trying to mark?

Jeff