Hi Jakub.
> On Thu, Dec 08, 2022 at 11:59:44AM +0100, Jose E. Marchesi via Gcc-patches
> wrote:
>> gcc/ChangeLog
>>
>> * expr.cc (expand_expr_divmod): Avoid side-effects of trying
>> sequences involving funcalls in optimization.
>
> That looks wrong.
> The globals for mentioned calls just shouldn't be emitted during expansion,
> especially if it is bigger annoyance than just having some extra symbols
> in the symbol table.
> expand_expr_divmod is definitely not the only place where something is
> expanded and later not used, lots of other places in the expander do that,
> and more importantly, there are over 80 optimization passes after expansion,
> many of them can remove code determined to be dead, and while lots of dead
> code is removed in GIMPLE optimizations already, definitely not all.
> So, rather than add hacks for this in a single spot, much better is to emit
> the globals only for stuff that is actually needed (so during final or
> immediately before it).
Yeah I see the point.
The culprit of the leadked .global seems to be a call to
assemble_external_libcall in emit_library_call_value_1:
expand_expr_divmod
expand_divmod -> This will result in libcall
sign_expand_divmod
emit_library_call_value
emit_library_call_value_1
...
/* If this machine requires an external definition for library
functions, write one out. */
assemble_external_libcall (fun);
...
The documented purpose of assemble_external_libcall is, as stated in
output.h, to "Assemble a string constant".
So, it seems to me that emit_library_call_value should not assemble
anything, since it is used by expand functions whose expansions may be
eventually discarded.
However, simply removing that call to assemble_external_libcall makes
.global declarations to not be emitted even when the funcall is actually
emitted in final:
For:
int foo(unsigned int len)
{
return ((long)len) * 234 / 5;
}
we get:
.file "foo.c"
.text
<------------- NO .global __divdi3
.align 3
.global foo
.type foo, @function
foo:
mov32 %r1,%r1
mov %r2,5
mul %r1,234
call __divdi3
exit
.size foo, .-foo
.ident "GCC: (GNU) 13.0.0 20221207 (experimental)"
Note that BPF lacks signed division instructions.
So, I guess the right fix would be to call assemble_external_libcall
during final? The `.global FOO' directive would be generated
immediately before the call sequence, but I guess that would be ok.
WDYT?
