57c23b967e
In some cases natural use of abstraction in avo programs can lead to redundant move instructions, specifically self-moves such as MOVQ AX, AX. This does not produce incorrect code but it is incorrect and inelegant. This diff introduces a PruneSelfMoves pass that removes such unnecessary instructions. Closes #76
51 lines
1.3 KiB
Go
51 lines
1.3 KiB
Go
package pass
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import (
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"github.com/mmcloughlin/avo/ir"
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"github.com/mmcloughlin/avo/operand"
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)
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// PruneSelfMoves removes move instructions from one register to itself.
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func PruneSelfMoves(fn *ir.Function) error {
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return removeinstructions(fn, func(i *ir.Instruction) bool {
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switch i.Opcode {
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case "MOVB", "MOVW", "MOVL", "MOVQ":
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default:
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return false
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}
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return operand.IsRegister(i.Operands[0]) && operand.IsRegister(i.Operands[1]) && i.Operands[0] == i.Operands[1]
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})
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}
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// removeinstructions deletes instructions from the given function which match predicate.
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func removeinstructions(fn *ir.Function, predicate func(*ir.Instruction) bool) error {
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// Removal of instructions has the potential to invalidate CFG structures.
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// Clear them to prevent accidental use of stale structures after this pass.
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invalidatecfg(fn)
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for i := 0; i < len(fn.Nodes); i++ {
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n := fn.Nodes[i]
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inst, ok := n.(*ir.Instruction)
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if !ok || !predicate(inst) {
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continue
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}
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copy(fn.Nodes[i:], fn.Nodes[i+1:])
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fn.Nodes[len(fn.Nodes)-1] = nil
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fn.Nodes = fn.Nodes[:len(fn.Nodes)-1]
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}
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return nil
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}
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// invalidatecfg clears CFG structures.
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func invalidatecfg(fn *ir.Function) {
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fn.LabelTarget = nil
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for _, i := range fn.Instructions() {
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i.Pred = nil
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i.Succ = nil
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}
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}
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