88 lines
2.6 KiB
Go
88 lines
2.6 KiB
Go
package optimizer
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// passRegDead eliminates dead stores to REGISTER variables.
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// Two-phase approach:
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// 1. Global pre-scan: if no lda regVar exists anywhere in the input,
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// all stores to that variable are provably dead (labels are irrelevant).
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// 2. Per-store local scan: for variables that DO have at least one lda,
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// scan forward through the current basic block.
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//
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// This is safe because REGISTER variables have a contract: nothing external
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// can observe their memory location (ASM/Script/Macro references are compile errors).
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func passRegDead(lines []asmLine, registerVars map[string]bool) []asmLine {
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// Phase 1 — global pre-scan: which REGISTER vars have at least one lda?
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hasLda := make(map[string]bool, len(registerVars))
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for _, l := range lines {
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if l.isCode && l.opcode == "lda" && l.operand != "" && registerVars[l.operand] {
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hasLda[l.operand] = true
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}
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}
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// Phase 2 — per-store decision
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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 !hasLda[line.operand] {
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// Globally dead — no lda anywhere in the program
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continue
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}
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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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