Re: [PATCH] bfin: Popcount-related improvements to machine description.

[Jeff Law via Gcc-patches]

On 10/17/2021 7:08 AM, Roger Sayle wrote:

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  <ro...@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.

OK
jeff