71 lines
2 KiB
Go
71 lines
2 KiB
Go
package optimizer
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// passRegDead eliminates dead stores to REGISTER variables.
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// A store is dead if no matching load (lda regVar) follows before A is
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// clobbered or a label is reached. This is safe because REGISTER variables
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// have a contract: nothing external can observe their memory location.
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func passRegDead(lines []asmLine, registerVars map[string]bool) []asmLine {
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var result []asmLine
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for i := 0; i < len(lines); i++ {
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line := lines[i]
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if line.isCode && line.opcode == "sta" && line.operand != "" && registerVars[line.operand] {
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if isRegStoreDead(lines, i+1, line.operand) {
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continue
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}
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}
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result = append(result, line)
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}
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return result
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}
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// isRegStoreDead scans forward from start to determine whether a store to a
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// REGISTER variable operand is dead. Returns true if the store can be removed.
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// A store is dead only if no matching lda is found anywhere forward before
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// a label or control-flow-ending instruction.
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func isRegStoreDead(lines []asmLine, start int, operand string) bool {
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for i := start; i < len(lines); i++ {
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l := lines[i]
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if l.optMarker || l.isComment {
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continue
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}
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if l.isLabel {
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return false
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}
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if l.isCode && isCallSite(l) {
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return false // callee may modify A; conservatively keep the store
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}
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if l.isCode && blocksFlow(l) {
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return true // rts/jmp/brk/rti — execution ends here
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}
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if l.isCode && l.opcode == "lda" && l.operand == operand {
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return false // the value IS read from memory later
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}
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}
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return true // end of block reached with no matching load
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}
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// blocksFlow returns true for instructions that unconditionally end the current
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// execution path: rts, jmp, brk, rti
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func blocksFlow(line asmLine) bool {
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switch line.opcode {
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case "rts", "jmp", "brk", "rti":
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return true
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}
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return false
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}
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// isCallSite returns true for instructions that transfer control to a callee
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// that may modify A, X, Y. The store before a call is conservatively kept.
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func isCallSite(line asmLine) bool {
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return line.opcode == "jsr"
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}
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