[middle-end PATCH] Prefer PLUS over IOR in RTL expansion of multi-word shifts/rotates.

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?

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.

0 files changed, 0 insertions, 0 deletions