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robfinch |
// ============================================================================
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// __
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// \\__/ o\ (C) 2017-2018 Robert Finch, Waterloo
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// \ __ / All rights reserved.
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// \/_// robfinch<remove>@finitron.ca
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// ||
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//
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// CC64 - 'C' derived language compiler
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// - 64 bit CPU
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//
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// This source file is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published
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// by the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This source file is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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//
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// ============================================================================
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//
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#include "stdafx.h"
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//
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// Returns the desirability of optimization for a subexpression.
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//
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// Immediate constants have low priority because small constants
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// can be directly encoded in the instruction. There's no value to
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// placing them in registers.
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int CSE::OptimizationDesireability()
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{
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if (exp==nullptr)
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return (0);
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if( voidf || (exp->nodetype == en_icon &&
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exp->i < 32768 && exp->i >= -32768))
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return (0);
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/* added this line to disable register optimization of global variables.
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The compiler would assign a register to a global variable ignoring
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the fact that the value might change due to a subroutine call.
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*/
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if (exp->nodetype == en_nacon)
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return (0);
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// No value to optimizing function call names, the called function
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// address will typically fit in a single 32/48 bit opcode. It's faster
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// to call a fixed label rather than an address in a register, because
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// the address is known at the fetch phase of the processor. A register
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// based address may be predicted by the processor and so is almost as
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// fast. Storing the address in a register can reduce the size of code.
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// If the function name can be optimized to a register then
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// a 16-bit compressed JAL can be used.
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if (exp->nodetype == en_cnacon && !opt_size)
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return (0);
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if (exp->isVolatile)
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return (0);
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// Prevent Inline code from being allocated a pointer in a register.
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if (exp->sym) {
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if (exp->sym->fi) {
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if (exp->sym->fi->IsInline)
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return (0);
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}
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}
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// Left values are worth more to optimization than right values.
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if( IsLValue(exp) )
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return (2 * uses);
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return (uses);
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}
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