Am 18.01.24 um 20:54 schrieb Roger Sayle:
This patch tweaks RTL expansion of multi-word shifts and rotates to use
PLUS rather than IOR for disjunctive operations. During expansion of
these operations, the middle-end creates RTL like (X<<C1) | (Y>>C2)
where the constants C1 and C2 guarantee that bits don't overlap.
Hence the IOR can be performed by any any_or_plus operation, such as
IOR, XOR or PLUS; for word-size operations where carry chains aren't
an issue these should all be equally fast (single-cycle) instructions.
The benefit of this change is that targets with shift-and-add insns,
like x86's lea, can benefit from the LSHIFT-ADD form.
An example of a backend that benefits is ARC, which is demonstrated
by these two simple functions:
But there are also back-ends where this is bad.
The reason is that with ORI, the back-end needs only to operate no
these sub-words where the sub-mask is non-zero. But for PLUS this
is not the case because the back-end does not know that intermediate
carry will be zero. Hence, with PLUS, more instructions are needed.
An example is AVR, but maybe much more target with multi-word operations
are affected in a bad way.
Take for example the case with 2 words and a value of 1.
LO |= 1
HI |= 0
can be optimized to
LO |= 1
but for addition this is not the case:
LO += 1
HI +=c 0 ;; Does not know that always carry = 0.
Johann
unsigned long long foo(unsigned long long x) { return x<<2; }
which with -O2 is currently compiled to:
foo: lsr r2,r0,30
asl_s r1,r1,2
asl_s r0,r0,2
j_s.d [blink]
or_s r1,r1,r2
with this patch becomes:
foo: lsr r2,r0,30
add2 r1,r2,r1
j_s.d [blink]
asl_s r0,r0,2
unsigned long long bar(unsigned long long x) { return (x<<2)|(x>>62); }
which with -O2 is currently compiled to 6 insns + return:
bar: lsr r12,r0,30
asl_s r3,r1,2
asl_s r0,r0,2
lsr_s r1,r1,30
or_s r0,r0,r1
j_s.d [blink]
or r1,r12,r3
with this patch becomes 4 insns + return:
bar: lsr r3,r1,30
lsr r2,r0,30
add2 r1,r2,r1
j_s.d [blink]
add2 r0,r3,r0
This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check, both with and without --target_board=unix{-m32}
with no new failures. Ok for mainline?
2024-01-18 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* expmed.cc (expand_shift_1): Use add_optab instead of ior_optab
to generate PLUS instead or IOR when unioning disjoint bitfields.
* optabs.cc (expand_subword_shift): Likewise.
(expand_binop): Likewise for double-word rotate.
Thanks in advance,
Roger
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