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125 lines
3.5 KiB
Text
125 lines
3.5 KiB
Text
pattern muldiv_c
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//
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// Authored by Akash Levy and Alain Dargelas of Silimate, Inc. under ISC license.
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// Transforms mul->div into const->mul when b and c are divisible constants:
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// y = (a * b_const) / c_const ===> a * eval(b_const / c_const)
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//
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state <SigSpec> a b_const mul_y
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match mul
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// Select multiplier
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select mul->type == $mul
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endmatch
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code a b_const mul_y
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// Get multiplier signals
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a = port(mul, \A);
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b_const = port(mul, \B);
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mul_y = port(mul, \Y);
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// Fanout of each multiplier Y bit should be 1 (no bit-split)
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if (nusers(mul_y) != 2)
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reject;
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// A and B can be interchanged
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branch;
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std::swap(a, b_const);
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endcode
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match div
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// Select div of form (a * b_const) / c_const
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select div->type == $div
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// Check that b_const and c_const is constant
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filter b_const.is_fully_const()
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filter port(div, \B).is_fully_const()
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index <SigSpec> remove_bottom_padding(port(div, \A)) === mul_y
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endmatch
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code
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// Get div signals
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SigSpec div_a = port(div, \A);
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SigSpec c_const = port(div, \B);
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SigSpec div_y = port(div, \Y);
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// Get offset of multiplier result chunk in divider
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int offset = GetSize(div_a) - GetSize(mul_y);
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// Get properties and values of b_const and c_const
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// b_const may be coming from the A port
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// But it is an RTLIL invariant that A_SIGNED equals B_SIGNED
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bool b_const_signed = mul->getParam(ID::B_SIGNED).as_bool();
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bool c_const_signed = div->getParam(ID::B_SIGNED).as_bool();
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int b_const_int = b_const.as_int(b_const_signed);
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int c_const_int = c_const.as_int(c_const_signed);
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int b_const_int_shifted = b_const_int << offset;
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// Helper lambdas for two's complement math
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auto sign2sComplement = [](auto value, int numBits) {
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if (value & (1 << (numBits - 1))) {
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return -1;
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} else {
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return 1;
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}
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};
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auto twosComplement = [](auto value, int numBits) {
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if (value & (1 << (numBits - 1))) {
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return (~value) + 1; // invert bits before adding 1
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} else {
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return value;
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}
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};
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// Two's complement conversion
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if (b_const_signed)
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b_const_int = sign2sComplement(b_const_int, GetSize(b_const)) * twosComplement(b_const_int, GetSize(b_const));
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if (c_const_signed)
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c_const_int = sign2sComplement(c_const_int, GetSize(c_const)) * twosComplement(c_const_int, GetSize(c_const));
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// Calculate the constant and compress the width to fit the value
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Const const_ratio;
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Const b_const_actual;
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// Avoid division by zero
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if (c_const_int == 0)
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reject;
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b_const_actual = b_const_int_shifted;
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b_const_actual.compress(b_const_signed);
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const_ratio = b_const_int_shifted / c_const_int;
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const_ratio.compress(b_const_signed | c_const_signed);
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// Integer values should be lesser than 32 bits
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// This is because we are using C++ types, and int is 32 bits
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// FIXME: use long long or BigInteger to make pass work with >32 bits
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if (GetSize(mul->getParam(ID::B_WIDTH)) > 32)
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reject;
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if (GetSize(b_const) > 32)
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reject;
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if (GetSize(c_const) + offset > 32)
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reject;
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// Check for potential multiplier overflow
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if (GetSize(b_const_actual) + GetSize(a) > GetSize(mul_y))
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reject;
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// Check that there are only zeros before offset
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if (offset < 0 || !div_a.extract(0, offset).is_fully_zero())
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reject;
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// Check that b is divisible by c
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if (b_const_int_shifted % c_const_int != 0)
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reject;
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// Rewire to only keep multiplier
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mul->setPort(\A, a);
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mul->setPort(\B, const_ratio);
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mul->setPort(\Y, div_y);
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// Remove divider
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autoremove(div);
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// Log, fixup, accept
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log("muldiv_const pattern in %s: mul=%s, div=%s\n", log_id(module), log_id(mul), log_id(div));
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mul->fixup_parameters();
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accept;
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endcode
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