On 6/6/19 10:42 PM, Alex Hill via gcc-help wrote:
>
> I need to reduce amount of branches in code. There is exist benchmark called median it have some code like:
> if ( A < B )
> return A = foo[i];
> else
> return B = foo[i];
> I wrote a pattern in machine description file *.md to avoid branches:
> (define_insn "smin<GPR:mode>3"
> [
> (set
> (match_operand:GPR 0 "register_operand" "=r")
> (if_then_else:GPR
> (lt:GPR
> (match_operand:GPR 1 "register_operand" " r")
> (match_operand:GPR 2 "register_operand" " r"))
> (match_dup 1)
> (match_dup 2)))
> ]
> ""
> "min\t%0,%1,%2"
> [(set_attr "type" "move")
> (set_attr "mode" "<MODE>")])
> It works in case of simple comparison:
> if ( A < B )
> return A ;
> else
> return B;
> GCC emit:
> min a0,a0,a1 # 9 smindi3 [length = 4]
> ret # 21 simple_return [length = 4]
> But if i try same, but with indexed variable( array ): it won`t works:
> if ( A < B )
> return A = foo[i];
> else
> return B = foo[i];
> GCC emit:
> blt a0,a1,.L5 # 11 *branch_orderdi [length = 4]
> sd a1,8(a2) # 18 *movdi_64bit/4 [length = 4]
> mv a0,a1 # 8 *movdi_64bit/1 [length = 4]
> ret # 34 simple_return [length = 4]
> .L5:
> sd a0,8(a2) # 13 *movdi_64bit/4 [length = 4]
> ret # 28 simple_return [length = 4]
> I need to GCC emit something like this:
> min a0,a0,a1 # 9 smindi3 [length = 4]
> sd a0,8(a2) # 18 *movdi_64bit/4 [length = 4]
> ret # 34 simple_return [length = 4]
> I appreciate any help, and i desperate to find a desicion, i tried to look into GIMPLE to change conditions of MIN_EXPR, but there is no(almost) any documentation, and sources are VERY puzzling((
Rather than using a match_dup which requires the exact same register
throughout the RTL pipeline, you might consider using a matching constraint.
(define_insn "smin<GPR:mode>3"
[
(set
(match_operand:GPR 0 "register_operand" "=r")
(if_then_else:GPR
(lt:GPR
(match_operand:GPR 1 "register_operand" " r")
(match_operand:GPR 2 "register_operand" " r"))
(match_operand:GPR 3 "register_operand" "1")
(match_operand:GPR 4 "register_operand" "2")))
I would also strongly recommnend reviewing the various dump files to see
if you're getting min/max as you leave gimple and if they're carried
through the RTL pipeline.
jeff
