Blackfin processors support a ONES instruction that implements a
32-bit popcount returning a 16-bit result.  This instruction was
previously described by GCC's bfin backed using a UNSPEC, but with
this patch uses a POPCOUNT:SI rtx to capture the semantics, allowing
it to evaluated at compile-time.  I've decided to keep the instruction
name the same (avoiding any changes to the __builtin_bfin_ones
machinery), but have provided popcountsi2 and popcounthi2 expanders
so that the middle-end can use this instruction to implement
__builtin_popcount (and __builtin_parity).

The new testcase ones.c
short foo ()
{
  int t = 5;
  short r = __builtin_bfin_ones(t);
  return r;
}

previously generated:
_foo:   nop;
        nop;
        R0 = 5 (X);
        R0.L = ONES R0;
        rts;

with this patch, now generates:
_foo:   nop;
        nop;
        nop;
        R0 = 2 (X);
        rts;

The new testcase popcount.c
int foo(int x)
{
  return __builtin_popcount(x);
}

previously generated:
_foo:   [--SP] = RETS;
        SP += -12;
        call ___popcountsi2;
        SP += 12;
        RETS = [SP++];
        rts;

now generates:
_foo:   nop;
        nop;
        R0.L = ONES R0;
        R0 = R0.L (Z);
        rts;

And the new testcase parity.c
int foo(int x)
{
  return __builtin_parity(x);
}

previously generated:
_foo:   [--SP] = RETS;
        SP += -12;
        call ___paritysi2;
        SP += 12;
        RETS = [SP++];
        rts;

now generates:
_foo:   nop;
        R1 = 1 (X);
        R0.L = ONES R0;
        R0 = R1 & R0;
        rts;


This patch has been tested on a cross-compiler to bfin-elf hosted
on x86_64-pc-linux-gnu, but without a toolchain, and shows no
regressions in the compile-only parts of the testsuite.
Ok for mainline?


2021-10-17  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/bfin/bfin.md (define_constants): Remove UNSPEC_ONES.
	(define_insn "ones"): Replace UNSPEC_ONES with a truncate of
	a popcount, allowing compile-time evaluation/simplification.
	(popcountsi2, popcounthi2): New expanders using a "ones" insn.

gcc/testsuite/ChangeLog
	* gcc.target/bfin/ones.c: New test case.
	* gcc.target/bfin/parity.c: New test case.
	* gcc.target/bfin/ones.c: New test case.

Thanks in advance,
Roger
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