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peepopt: fix and refactor `shiftadd` 

- moved all selection and filtering logic to the match block
- applied less-verbose code suggestions
- removed constraint on number of bits in shift-amount
- added check for possible wrap-arround in the operation
This commit is contained in:
phsauter 2023-11-06 14:01:37 +01:00
parent 72c6a01e67
commit 9ca57d9f13
1 changed files with 38 additions and 34 deletions
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72
passes/pmgen/peepopt_shiftadd.pmg
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@ -35,15 +35,12 @@ code shift_amount log2scale msb_zeros
shift_amount.remove(GetSize(shift_amount) - 1);
if (shift_amount.empty()) reject;
}
if (GetSize(shift_amount) > 20)
reject;
endcode
state <SigSpec> add_var
state <Const> add_const
state <bool> is_sub
state <bool> varport_A
state <bool> var_signed
state <SigSpec> var_signal
// offset: signed constant value c in data[var+c +:W1] (constant shift-right amount)
state <int> offset
match add
// either data[var+c +:W1] or data[var-c +:W1]
@ -52,39 +49,46 @@ match add
// one must be constant, the other is variable
choice <IdString> constport {\A, \B}
filter port(add, constport).is_fully_const()
select !port(add, constport).empty()
select port(add, constport).is_fully_const()
define <IdString> varport (constport == \A ? \B : \A)
set is_sub add->type.in($sub)
set varport_A (varport == \A)
// if a value of var is able to wrap the output, the transformation might give wrong results
// an addition/substraction can at most flip one more bit than the largest operand (the carry bit)
// as long as the output can show this bit, no wrap should occur (assuming all signed-ness make sense)
select ( GetSize(port(add, \Y)) > max(GetSize(port(add, \A)), GetSize(port(add, \B))) )
// (var+c)<<N -> (var<<N)+(c<<N), also true for signed values
set add_const SigSpec({port(add, constport), SigSpec(State::S0, log2scale)}).as_const()
define <bool> varport_A (varport == \A)
define <bool> is_sub add->type.in($sub)
// get add_var unmapped (so no `port()` shorthand)
// to attach it to the transformed shift cells \B port
set add_var add->getPort(varport)
define <bool> constport_signed param(add, !varport_A ? \A_SIGNED : \B_SIGNED).as_bool()
define <bool> varport_signed param(add, varport_A ? \A_SIGNED : \B_SIGNED).as_bool();
define <bool> offset_negative ((port(add, constport).bits().back() == State::S1) ^ (is_sub && varport_A))
// checking some value boundaries as well:
// data[...-c +:W1] is fine for +/-var (pad at LSB, all data still accessible)
// data[...+c +:W1] is only fine for +var(add) and var unsigned
// (+c cuts lower C bits, making them inaccessible, a signed var could try to access them)
// either its an add or the variable port is A (it must be positive)
select (add->type.in($add) || varport == \A)
// -> data[var+c +:W1] (with var signed) is illegal
filter !(!offset_negative && varport_signed)
// state-variables are assigned at the end only:
// shift the log2scale offset in-front of add to get true value: (var+c)<<N -> (var<<N)+(c<<N)
set offset ( (port(add, constport).as_int(constport_signed) << log2scale) * ( (is_sub && varport_A) ? -1 : 1 ) )
set var_signed varport_signed
set var_signal add->getPort(varport)
endmatch
code
{
log_debug("shiftadd candidate in %s: shift=%s, add/sub=%s\n", log_id(module), log_id(shift), log_id(add));
if (add_const.empty() || GetSize(add_const) > 20)
reject;
int offset = add_const.as_int() * ( (is_sub && varport_A) ? -1 : 1 );
bool varport_signed = (varport_A && param(add, \A_SIGNED).as_bool()) \
|| (!varport_A && param(add, \B_SIGNED).as_bool());
// data[...-c +:W1] is fine for +/-var (pad at LSB, all data still accessible)
// data[...+c +:W1] is only fine for +var(add) and var unsigned
// (+c cuts lower C bits, making them inaccessible, a signed var could try to access them)
// -> data[c-var +:W1] is illegal
if (is_sub && !varport_A)
reject;
// -> data[var+c +:W1] (with var signed) is illegal
if ( (offset>0) && varport_signed )
if (offset>0 && var_signed) {
log("I should not be here %x\n", var_signed);
reject;
}
did_something = true;
log("shiftadd pattern in %s: shift=%s, add/sub=%s, offset: %d\n", \
@ -98,12 +102,12 @@ code
new_a.append(old_a);
} else {
// data >> (...+c) transformed to data[MAX:c] >> (...)
new_a.append(old_a.extract(offset, GetSize(old_a)-1-offset));
new_a.append(old_a.extract_end(offset));
}
SigSpec new_b = {add_var, SigSpec(State::S0, log2scale)};
if (msb_zeros || !varport_signed)
SigSpec new_b = {var_signal, SigSpec(State::S0, log2scale)};
if (msb_zeros || !var_signed)
new_b.append(State::S0);
shift->setPort(\A, new_a);