[PATCH] RISC-V: Adjust tt-ascalon-d8 branch cost

[Anton Blanchard]

If-conversion isn't being applied to this nbench code:

 #include <stdint.h>
 #define INTERNAL_FPF_PRECISION 4
 typedef uint16_t u16;

 void ShiftMantLeft1(u16 *carry, u16 *mantissa)
 {
         int i;
         int new_carry;
         u16 accum;

         for(i=INTERNAL_FPF_PRECISION-1;i>=0;i--)
         {       accum=mantissa[i];
                 new_carry=accum & 0x8000;
                 accum=accum<<1;
                 if(*carry)
                         accum|=1;
                 *carry=new_carry;
                 mantissa[i]=accum;
         }
         return;
 }

Bumping branch_cost from 3 to 4 triggers if-conversion, improving the
nbench FP EMULATION result on Ascalon significantly. There's a risk
that more aggressive use of conditional zero instructions will negatively
impact workloads that predict well, but we haven't seen anything obvious.

gcc/ChangeLog:
        * config/riscv/riscv.cc (tt_ascalon_d8_tune_info): Increase branch_cost
        from 3 to 4.

Signed-off-by: Anton Blanchard <ant...@tenstorrent.com>
---
 gcc/config/riscv/riscv.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index 07d40f459e3..bfd43fba101 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -659,7 +659,7 @@ static const struct riscv_tune_param 
tt_ascalon_d8_tune_info = {
   {COSTS_N_INSNS (3), COSTS_N_INSNS (3)},      /* int_mul */
   {COSTS_N_INSNS (13), COSTS_N_INSNS (13)},    /* int_div */
   8,                                           /* issue_rate */
-  3,                                           /* branch_cost */
+  4,                                           /* branch_cost */
   4,                                           /* memory_cost */
   4,                                           /* fmv_cost */
   false,                                       /* slow_unaligned_access */
-- 
2.34.1