> James Greenhalgh wrote: > On Mon, Apr 27, 2015 at 02:42:36PM +0100, Wilco Dijkstra wrote: > > > -----Original Message----- > > > From: Wilco Dijkstra [mailto:wdijk...@arm.com] > > > Sent: 03 March 2015 16:19 > > > To: GCC Patches > > > Subject: [PATCH][AArch64] Use conditional negate for abs expansion > > > > > > Expand abs into a compare and conditional negate. This is the most > > > obvious expansion, > enables > > > merging of the comparison into ALU instructions and is faster on all > > > implementations. > > > Bootstrapped & regression tested. > > > > > > int f(int x) { return abs (x + 1); } > > > > > > Before: > > > add w0, w0, 1 > > > sxtw x0, w0 > > > eor x1, x0, x0, asr 63 > > > sub x1, x1, x0, asr 63 > > > mov x0, x1 > > > ret > > > > > > After: > > > adds w0, w0, 1 > > > csneg w0, w0, w0, pl > > > ret > > > > > > ChangeLog: > > > > > > 2015-03-03 Wilco Dijkstra <wdijk...@arm.com> > > > > > > * gcc/config/aarch64/aarch64.md (absdi2): optimize abs expansion. > > > (csneg3<mode>_insn): enable expansion of pattern. > > > * gcc/testsuite/gcc.target/aarch64/abs_1.c (abs64): update test > > > for new abs expansion. (abs64_in_dreg): likewise. > > > This looks like it breaks support for abs in a D register (for example > at the end of a loop, or extracted from Neon Intrinsics, etc). > > e.g. (totally contrived...) > > int64x1_t > abs_max (int64x2_t x, int64_t *wb) > { > int64_t y = vgetq_lane_s64 (x, 0); > if (y < 0) > y = -y; > return vdup_n_s64 (y); > } > > Which currently generates: > > abs_max: > abs d0, d0 > ret > > I suppose we don't need to worry too much about that (and the current > implementation doesn't seem to catch it reliably anyway), but it would be > good if we could keep the support - even if it is rarely used.
Well it may be possible to write a peephole for this rare case, but I hope people would use a vabs if that's what they wanted... Wilco
