On 11/18/23 22:38, Maciej W. Rozycki wrote:
In the non-zero case there is no need for the conditional value used by
Ventana and Zicond integer conditional operations to be specifically 1.
Regardless we canonicalize it by producing an extraneous conditional-set
operation, such as with the sequence below:
(insn 22 6 23 2 (set (reg:DI 141)
(minus:DI (reg/v:DI 135 [ w ])
(reg/v:DI 136 [ x ]))) 11 {subdi3}
(nil))
(insn 23 22 24 2 (set (reg:DI 140)
(ne:DI (reg:DI 141)
(const_int 0 [0]))) 307 {*sne_zero_didi}
(nil))
(insn 24 23 25 2 (set (reg:DI 143)
(if_then_else:DI (eq:DI (reg:DI 140)
(const_int 0 [0]))
(const_int 0 [0])
(reg:DI 13 a3 [ z ]))) 27913 {*czero.eqz.didi}
(nil))
(insn 25 24 26 2 (set (reg:DI 142)
(if_then_else:DI (ne:DI (reg:DI 140)
(const_int 0 [0]))
(const_int 0 [0])
(reg/v:DI 137 [ y ]))) 27914 {*czero.nez.didi}
(nil))
(insn 26 25 18 2 (set (reg/v:DI 138 [ z ])
(ior:DI (reg:DI 142)
(reg:DI 143))) 105 {iordi3}
(nil))
where insn 23 can well be removed without changing the semantics of the
sequence. This is actually fixed up later on by combine and the insn
does not make it to output meaning no SNEZ (or SEQZ in the reverse case)
appears in the assembly produced, however it counts towards the cost of
the sequence calculated by if-conversion, raising the trigger level for
the branchless sequence to be chosen. Arguably to emit this extraneous
operation it can be also considered rather sloppy of our backend's.
Remove the check for operand 1 being constant 0 in the Ventana/Zicond
case for equality comparisons then, observing that `riscv_zero_if_equal'
called via `riscv_emit_int_compare' will canonicalize the comparison if
required, removing the extraneous insn from output:
(insn 22 6 23 2 (set (reg:DI 142)
(minus:DI (reg/v:DI 135 [ w ])
(reg/v:DI 136 [ x ]))) 11 {subdi3}
(nil))
(insn 23 22 24 2 (set (reg:DI 141)
(if_then_else:DI (eq:DI (reg:DI 142)
(const_int 0 [0]))
(const_int 0 [0])
(reg:DI 13 a3 [ z ]))) 27913 {*czero.eqz.didi}
(nil))
(insn 24 23 25 2 (set (reg:DI 140)
(if_then_else:DI (ne:DI (reg:DI 142)
(const_int 0 [0]))
(const_int 0 [0])
(reg/v:DI 137 [ y ]))) 27914 {*czero.nez.didi}
(nil))
(insn 25 24 18 2 (set (reg/v:DI 138 [ z ])
(ior:DI (reg:DI 140)
(reg:DI 141))) 105 {iordi3}
(nil))
while keeping actual assembly produced the same.
Adjust branch costs across the test cases affected accordingly.
gcc/
* config/riscv/riscv.cc (riscv_expand_conditional_move): Remove
the check for operand 1 being constant 0 in the Ventana/Zicond
case for equality comparisons.
gcc/testsuite/
*
gcc.target/riscv/zicond-primitiveSemantics_compare_imm_return_imm_imm.c:
Lower `-mbranch-cost=' setting.
*
gcc.target/riscv/zicond-primitiveSemantics_compare_imm_return_imm_reg.c:
Likewise.
*
gcc.target/riscv/zicond-primitiveSemantics_compare_imm_return_reg_reg.c:
Likewise.
*
gcc.target/riscv/zicond-primitiveSemantics_compare_reg_return_imm_imm.c:
Likewise.
*
gcc.target/riscv/zicond-primitiveSemantics_compare_reg_return_imm_reg.c:
Likewise.
*
gcc.target/riscv/zicond-primitiveSemantics_compare_reg_return_reg_reg.c:
Likewise.
OK. Thanks for catching this!
jeff
