From: lkcl <luke.leighton@gmail.com>
To: Alan Modra <amodra@gmail.com>,Dmitry Selyutin <ghostmansd@gmail.com>
Cc: Binutils <binutils@sourceware.org>
Subject: Re: Teaching expression() to treat some operations specially
Date: Mon, 11 Jul 2022 03:33:55 +0100 [thread overview]
Message-ID: <2A99A5EA-C863-465C-A983-244A48D2E3C3@gmail.com> (raw)
In-Reply-To: <Yst2WXcpFZ3udhPx@squeak.grove.modra.org>
alan thank you for the insights. i am using an unusual mailer which only topposts: rather than subject everyone to that i'm trimming all context.
some background on svp64 predication (which is not part of Power ISA): it is the same concept as RVV and NEC SX Aurora (Cray-style) predicate masks except a bit more sophisticated. basic concept of Predicated Cray Vectors: elements may be dynamically skipped at runtime based on VL bits read from the regiater file:
for i in range(VL):
if get_mask()&(1<<i) == 0: continue
// do operation
get_mask() above may be one of the following sources:
* all ones (disables predication)
* the contents of r3, r10, r31 or their bit-inverses
* 1<<r3 i.e. one single unary bit
* the concatenation (and inversion) of a selected CR Field
from CR0, CR1, ..... CRn
sm=eq means concatenate CR0.eq CR1.eq CR2.eq as the mask
i particularly like 1<<r3 because it is
for i in range(VL):
if (1<<GPR(r3))&(1<<i) == 0: continue
GPR(RT+i) = GPR(RA+i) + GPR(RB+i) # vector add
i.e. just
GPR(RT+GPR(r3)) = GPR(RA+GPR(r3)) + GPR(RB+GPR(r3))
this should help clear up some potential confusion on the meaning of sm=~r3, it is not the inversion of a static constant "3" (to create 0xfff....fffc) but a mnemonic to put a bitpattern into the 24-bit prefix, "at *instruction execute* time please read then use contents of register r3 as the predicate mask, invert all its bits before passing it on".
we could have used "sm=0b0111" or other numeric code and simply dropped that directly into the 24-bit prefix. however this loses the opportunity to perform macro substitution.
another idea we discussed was to move the "~" to the lhs:
~sm=r3 / !sm=r3
or
sm ~= r3
on the basis that only the rhs gets passed to expression(). in english:
inverted source mask is r3
readability of 1<<r3 is compromised however by trying something aimilar. we thought perhaps "sm=ur3" or such, for "source mask is unary from r3" but it is pushing things.
adapting 0_constant to recognise 1<<r3 as a special pattern would help preserve readability greatly. i take it you mean that after the lexer has recognised "1" "<<" "r3" that there is a tree-walk done by cr_operand() that *replaces* the patterns?
l.
next prev parent reply other threads:[~2022-07-11 2:34 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-09 16:47 Dmitry Selyutin
2022-07-11 1:01 ` Alan Modra
2022-07-11 2:33 ` lkcl [this message]
2022-07-11 3:08 ` Alan Modra
2022-07-11 14:28 ` Dmitry Selyutin
2022-07-11 19:13 ` Dmitry Selyutin
2022-07-11 23:37 ` Alan Modra
2022-07-12 4:18 ` Dmitry Selyutin
2022-07-12 6:25 ` Jan Beulich
2022-07-12 6:47 ` Dmitry Selyutin
2022-07-12 6:57 ` Jan Beulich
2022-07-12 8:50 ` Dmitry Selyutin
2022-07-12 10:44 ` Jan Beulich
2022-07-12 11:17 ` Dmitry Selyutin
2022-07-12 11:39 ` Jan Beulich
2022-07-12 9:01 ` Dmitry Selyutin
2022-07-12 6:58 ` Dmitry Selyutin
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