On Tue, Apr 29, 2025 at 4:25 PM Richard Biener
<richard.guent...@gmail.com> wrote:
>
> On Tue, Apr 29, 2025 at 9:39 AM H.J. Lu <hjl.to...@gmail.com> wrote:
> >
> > For targets, like x86, which define TARGET_PROMOTE_PROTOTYPES to return
> > true, all integer arguments smaller than int are passed as int:
> >
> > [hjl@gnu-tgl-3 pr14907]$ cat x.c
> > extern int baz (char c1);
> >
> > int
> > foo (char c1)
> > {
> > return baz (c1);
> > }
> > [hjl@gnu-tgl-3 pr14907]$ gcc -S -O2 -m32 x.c
> > [hjl@gnu-tgl-3 pr14907]$ cat x.s
> > .file "x.c"
> > .text
> > .p2align 4
> > .globl foo
> > .type foo, @function
> > foo:
> > .LFB0:
> > .cfi_startproc
> > movsbl 4(%esp), %eax
> > movl %eax, 4(%esp)
> > jmp baz
> > .cfi_endproc
> > .LFE0:
> > .size foo, .-foo
> > .ident "GCC: (GNU) 14.2.1 20240912 (Red Hat 14.2.1-3)"
> > .section .note.GNU-stack,"",@progbits
> > [hjl@gnu-tgl-3 pr14907]$
> >
> > But integer promotion:
> >
> > movsbl 4(%esp), %eax
> > movl %eax, 4(%esp)
> >
> > isn't necessary if incoming arguments are copied to outgoing arguments
> > directly.
> >
> > Add a new target hook, TARGET_GET_SMALL_INTEGER_ARGUMENT_VALUE, defaulting
> > to return nullptr. If the new target hook returns non-nullptr, use it to
> > get the outgoing small integer argument. The x86 target hook returns the
> > value of the corresponding incoming argument as int if it can be used as
> > the outgoing argument. If callee is a global function, we always properly
> > extend the incoming small integer arguments in callee. If callee is a
> > local function, since DECL_ARG_TYPE has the original small integer type,
> > we will extend the incoming small integer arguments in callee if needed.
> > It is safe only if
> >
> > 1. Caller and callee are not nested functions.
> > 2. Caller and callee use the same ABI.
>
> How do these influence the value? TARGET_PROMOTE_PROTOTYPES
> should apply to all of them, no?
When the arguments are passed in different registers in different ABIs,
we have to copy them anyway.
>
> > 3. The incoming argument and the outgoing argument are in the same
> > location.
>
> Why's that? Can't we move them but still elide the sign-/zero-extension?
If they aren't in the same locations, we have to move them anyway.
This patch tries to avoid necessary moves of incoming arguments to
outgoing arguments.
> > 4. The incoming argument is unchanged before call expansion.
>
> Obviously, but then IMO this reveals an issue with the design of a target hook
> returning the argument register - it returns a place rather than a
> value. Wha'ts
We need the place so that we can avoid meaningless copy.
> the limitation of implementing this without help of the target?
Middle-end may not know what is safe and not safe, for example, we
can skip the hidden argument SUBREG for x32.
> Richard.
>
> > Otherwise, using the incoming argument as the outgoing argument may change
> > values of other incoming arguments or the wrong outgoing argument value
> > may be used.
> >
> > gcc/
> >
> > PR middle-end/14907
> > * calls.cc (arg_data): Add small_integer_argument_value.
> > (precompute_register_parameters): Set args[i].value to
> > args[i].small_integer_argument_value if not nullptr.
> > (initialize_argument_information): Set
> > args[i].small_integer_argument_value to
> > TARGET_GET_SMALL_INTEGER_ARGUMENT_VALUE.
> > (store_one_arg): Set arg->value to arg->small_integer_argument_value
> > if not nullptr.
> > * target.def (get_small_integer_argument_value): New for calls.
> > * targhooks.cc (default_get_small_integer_argument_value): New.
> > * targhooks.h (default_get_small_integer_argument_value): Likewise.
> > * config/i386/i386.cc (ix86_get_small_integer_argument_value): New.
> > (TARGET_GET_SMALL_INTEGER_ARGUMENT_VALUE): Likewise.
> > * config/i386/i386.h (machine_function): Add
> > no_small_integer_argument_value and before_first_expand_call.
> > * doc/tm.texi: Regenerated.
> > * doc/tm.texi.in (TARGET_GET_SMALL_INTEGER_ARGUMENT_VALUE): New
> > hook.
> >
> > gcc/testsuite/
> >
> > PR middle-end/14907
> > * gcc.target/i386/pr14907-1.c: New test.
> > * gcc.target/i386/pr14907-2.c: Likewise.
> > * gcc.target/i386/pr14907-3.c: Likewise.
> > * gcc.target/i386/pr14907-4.c: Likewise.
> > * gcc.target/i386/pr14907-5.c: Likewise.
> > * gcc.target/i386/pr14907-6.c: Likewise.
> > * gcc.target/i386/pr14907-7a.c: Likewise.
> > * gcc.target/i386/pr14907-7b.c: Likewise.
> > * gcc.target/i386/pr14907-8a.c: Likewise.
> > * gcc.target/i386/pr14907-8b.c: Likewise.
> > * gcc.target/i386/pr14907-9a.c: Likewise.
> > * gcc.target/i386/pr14907-9b.c: Likewise.
> > * gcc.target/i386/pr14907-10a.c: Likewise.
> > * gcc.target/i386/pr14907-10b.c: Likewise.
> > * gcc.target/i386/pr14907-10c.c: Likewise.
> > * gcc.target/i386/pr14907-11.c: Likewise.
> > * gcc.target/i386/pr14907-12.c: Likewise.
> > * gcc.target/i386/pr14907-13.c: Likewise.
> > * gcc.target/i386/pr14907-14.c: Likewise.
> > * gcc.target/i386/pr14907-15.c: Likewise.
> > * gcc.target/i386/pr14907-16.c: Likewise.
> > * gcc.target/i386/pr14907-17.c: Likewise.
> > * gcc.target/i386/pr14907-18a.c: Likewise.
> > * gcc.target/i386/pr14907-18b.c: Likewise.
> > * gcc.target/i386/pr14907-19.c: Likewise.
> > * gcc.target/i386/pr14907-20a.c: Likewise.
> > * gcc.target/i386/pr14907-20b.c: Likewise.
> > * gcc.target/i386/pr14907-21.c: Likewise.
> > * gcc.target/i386/pr14907-22.c: Likewise.
> >
> >
> > --
> > H.J.
--
H.J.
