On 1/18/24 12:54 PM, Roger Sayle wrote:
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:
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.
Also note that on some targets like RISC-V, there's more freedom to
generate compressed instructions from "and" rather than "or".
Anyway, given the time elapsed since submission, I went ahead and
retested on x86, then committed & pushed to the trunk.
Thanks!
jeff
