Hi Andrew,
On 12/09/15 02:15, Andrew Pinski wrote:
On Tue, Sep 1, 2015 at 6:08 PM, Kyrill Tkachov <kyrylo.tkac...@arm.com> wrote:
Hi all,
The ARMv8-A reference manual says:
"CNEG <Wd>, <Wn>, <cond>
is equivalent to
CSNEG <Wd>, <Wn>, <Wn>, invert(<cond>)
and is the preferred disassembly when Rn == Rm && cond != '111x'."
That is, when the two input registers are the same we can use the shorter
CNEG mnemonic
with the inverse condition instead of the longer CSNEG instruction.
Similarly for the
CSINV and CSINC instructions, they have shorter CINV and CINC forms.
This patch adjusts the output templates to emit the preferred shorter
sequences when possible.
The new mnemonics are just aliases, they map down to the same instruction in
the end, so there
are no performance or behaviour implications. But it does make the assembly
a bit more readable
IMO, since:
"cneg w27, w9, le"
can be simply read as "if the condition is less or equal negate w9" instead
of the previous:
"csneg w27, w9, w9, gt" where you have to remember which of the input
registers is negated.
Bootstrapped and tested on aarch64-linux-gnu.
Ok for trunk?
I really think this kind of special casing is not correct and does not
belong in the compiler. The main reason it complicates the back-end
more than the benefit of easier of reading the assembly code.
<sorry for the delay>.
The complication is an extra if-else statement with
explicit strings on each arm i.e. there's no snprintf trickery.
I tend to read a lot of the generated assembly when performing assembly
comparisons while working on performance patches and I find that having
the cneg from with two register operands and the negate condition is quicker
to parse than the full csneg form where I need to remember that extra bit
of info that the condition there must be inverted to get the negation condition.
If you feel very strongly against this I can withdraw this patch, but I'd
rather have it in.
FWIW, clang also emits the CNEG when it can AFAICS, though I admit that's not
necessarily a strong
argument for this change.
Kyrill
Thanks,
Andrew Pinski
Thanks,
Kyrill
2015-09-01 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* config/aarch64/aarch64.md (csinc3<mode>_insn): Use CINC
mnemonic when possible.
(*csinv3<mode>_insn): Use CINV mnemonic when possible.
(csneg3<mode>_insn): USE CNEG mnemonic when possible.
2015-09-01 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* gcc.target/aarch64/abs_1.c: Update scan-assembler checks
to allow cneg.
* gcc.target/aarch64/cond_op_imm_1.c: Likewise. Likewise for cinv.
* gcc.target/aarch64/mod_2.c: Likewise.
