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cmd/compile: make prove use non-equality in subtraction for a stronger bound #76609

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cmd/compile: make prove use non-equality in subtraction for a stronger bound #76609

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24 changes: 11 additions & 13 deletions src/cmd/compile/internal/ssa/prove.go
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Original file line number Diff line number Diff line change
Expand Up @@ -2162,24 +2162,22 @@ func (ft *factsTable) detectSubRelations(v *Value) {
return // x-y might overflow
}

// Subtracting a positive number only makes
// things smaller.
if yLim.min >= 0 {
// Subtracting a positive non-zero number only makes
// things smaller. If it's positive or zero, it might
// also do nothing (x-0 == v).
if yLim.min > 0 {
ft.update(v.Block, v, x, signed, lt)
} else if yLim.min == 0 {
ft.update(v.Block, v, x, signed, lt|eq)
// TODO: is this worth it?
//if yLim.min > 0 {
// ft.update(v.Block, v, x, signed, lt)
//}
}

// Subtracting a number from a bigger one
// can't go below 0.
if ft.orderS.OrderedOrEqual(y, x) {
// can't go below 1. If the numbers might be
// equal, then it can't go below 0.
if ft.orderS.Ordered(y, x) {
ft.signedMin(v, 1)
} else if ft.orderS.OrderedOrEqual(y, x) {
ft.setNonNegative(v)
// TODO: is this worth it?
//if ft.orderS.Ordered(y, x) {
// ft.signedMin(v, 1)
//}
}
}

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