On Mon, Sep 4, 2023 at 2:28 AM Hongtao Liu <crazy...@gmail.com> wrote:
> > > > > > > I think there should be some constraint which explicitly has all
> > > > > > > the 32
> > > > > > > GPRs, like there is one for just all 16 GPRs (h), so that
> > > > > > > regardless of
> > > > > > > -mapx-inline-asm-use-gpr32 one can be explicit what the inline
> > > > > > > asm wants.
> > > > > > >
> > > > > > > Also, what about the "g" constraint? Shouldn't there be another
> > > > > > > for "g"
> > > > > > > without r16..r31? What about the various other memory
> > > > > > > constraints ("<", "o", ...)?
> > > > > >
> > > > > > I think we should leave all existing constraints as they are, so "r"
> > > > > > covers only GPR16, "m" and "o" to only use GPR16. We can then
> > > > > > introduce "h" to instructions that have the ability to handle EGPR.
> > > > > > This would be somehow similar to the SSE -> AVX512F transition,
> > > > > > where
> > > > > > we still have "x" for SSE16 and "v" was introduced as a separate
> > > > > > register class for EVEX SSE registers. This way, asm will be
> > > > > > compatible, when "r", "m", "o" and "g" are used. The new memory
> > > > > > constraint "Bt", should allow new registers, and should be added to
> > > > > > the constraint string as a separate constraint, and conditionally
> > > > > > enabled by relevant "isa" (AKA "enabled") attribute.
> > > > >
> > > > > The extended constraint can work for registers, but for memory it is
> > > > > more
> > > > > complicated.
> > > >
> > > > Yes, unfortunately. The compiler assumes that an unchangeable register
> > > > class is used for BASE/INDEX registers. I have hit this limitation
> > > > when trying to implement memory support for instructions involving
> > > > 8-bit high registers (%ah, %bh, %ch, %dh), which do not support REX
> > > > registers, also inside memory operand. (You can see the "hack" in e.g.
> > > > *extzvqi_mem_rex64" and corresponding peephole2 with the original
> > > > *extzvqi pattern). I am aware that dynamic insn-dependent BASE/INDEX
> > > > register class is the major limitation in the compiler, so perhaps the
> > > > strategy on how to override this limitation should be discussed with
> > > > the register allocator author first. Perhaps adding an insn attribute
> > > > to insn RTX pattern to specify different BASE/INDEX register sets can
> > > > be a better solution than passing insn RTX to the register allocator.
> > > >
> > > > The above idea still does not solve the asm problem on how to select
> > > > correct BASE/INDEX register set for memory operands.
> > > The current approach disables gpr32 for memory operand in asm_operand
> > > by default. but can be turned on by options
> > > ix86_apx_inline_asm_use_gpr32(users need to guarantee the instruction
> > > supports gpr32).
> > > Only ~ 5% of total instructions don't support gpr32, reversed approach
> > > only gonna get more complicated.
> >
> > I'm not referring to the reversed approach, just want to point out
> > that the same approach as you proposed w.r.t. to memory operand can be
> > achieved using some named insn attribute that would affect BASE/INDEX
> > register class selection. The attribute could default to gpr32 with
> > APX, unless the insn specific attribute has e.g. nogpr32 value. See
> > for example how "enabled" and "preferred_for_*" attributes are used.
> > Perhaps this new attribute can also be applied to separate
> > alternatives.
> Yes, for xop/fma4/3dnow instructions, I think we can use isa attr like
> (define_attr "gpr32" "0, 1"
> (cond [(eq_attr "isa" "fma4")
> (const_string "0")]
> (const_string "1")))
Just a nit, can the member be named "map0" and "map1"? The code will
then look like:
if (get_attr_gpr32 (insn) == GPR32_MAP0) ...
instead of:
if (get_attr_gpr32 (insn) == GPR32_0) ...
> But still, we need to adjust memory constraints in the pattern.
I guess the gpr32 property is the same for all alternatives of the
insn pattern. In this case, "m" "g" and "a" constraints could remain
as they are, the final register class will be adjusted (by some target
hook?) based on the value of gpr32 attribute.
> Ideally, gcc includes encoding information for every instruction,
> (.i.e. map0/map1), so that we can determine the attribute value of
> gpr32 directly from this information.
I think the right tool for this is attribute infrastructure of insn
patterns. We can set the default, set precise value of the insns, or
calculate attribute from some other attribute in a quite flexible way.
Other than that, adjusting BASE/INDEX register class of the RA pass is
the infrastructure change, but perhaps similar to the one you
proposed.
Uros.
