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cmd/compile: compute Modu's maximum limits from argument's limits

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addLocalFacts loop already ft.update which sets up limits correctly, but doing this in flowLimit help us since other values might depend on this limit.

Updates #68857

We could improve this further:
- remove mod alltogheter when we can prove a < b.
- we could do more adhoc computation in flowLimit to set umax and umin tighter

Change-Id: I5184913577b6a51a07cb53a6e6b73552a982de0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/605156
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Jorropo 2024-08-13 17:17:06 +02:00 committed by Keith Randall
parent 57df33814a
commit 194fa2eb6c
2 changed files with 59 additions and 0 deletions
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7
src/cmd/compile/internal/ssa/prove.go
View File

@ -1775,6 +1775,11 @@ func (ft *factsTable) flowLimit(v *Value) bool {
a := ft.limits[v.Args[0].ID]
b := ft.limits[v.Args[1].ID]
return ft.newLimit(v, a.mul(b.exp2(8), 8))
case OpMod64u, OpMod32u, OpMod16u, OpMod8u:
a := ft.limits[v.Args[0].ID]
b := ft.limits[v.Args[1].ID]
// Underflow in the arithmetic below is ok, it gives to MaxUint64 which does nothing to the limit.
return ft.unsignedMax(v, minU(a.umax, b.umax-1))
case OpPhi:
// Compute the union of all the input phis.
@ -1909,6 +1914,8 @@ func addLocalFacts(ft *factsTable, b *Block) {
// Add facts about individual operations.
for _, v := range b.Values {
// FIXME(go.dev/issue/68857): this loop only set up limits properly when b.Values is in topological order.
// flowLimit can also depend on limits given by this loop which right now is not handled.
switch v.Op {
case OpAnd64, OpAnd32, OpAnd16, OpAnd8:
ft.update(b, v, v.Args[0], unsigned, lt|eq)

52
test/prove.go
View File

@ -1426,6 +1426,58 @@ func or64(a, b uint64, ensureBothBranchesCouldHappen bool) int {
return int(z)
}
func mod64uWithSmallerDividendMax(a, b uint64, ensureBothBranchesCouldHappen bool) int {
a &= 0xff
b &= 0xfff
z := bits.Len64(a % b) // see go.dev/issue/68857 for bits.Len64
if ensureBothBranchesCouldHappen {
if z > bits.Len64(0xff) { // ERROR "Disproved Less64$"
return 42
}
} else {
if z <= bits.Len64(0xff) { // ERROR "Proved Leq64$"
return 1337
}
}
return z
}
func mod64uWithSmallerDivisorMax(a, b uint64, ensureBothBranchesCouldHappen bool) int {
a &= 0xfff
b &= 0x10 // we need bits.Len64(b.umax) != bits.Len64(b.umax-1)
z := bits.Len64(a % b) // see go.dev/issue/68857 for bits.Len64
if ensureBothBranchesCouldHappen {
if z > bits.Len64(0x10-1) { // ERROR "Disproved Less64$"
return 42
}
} else {
if z <= bits.Len64(0x10-1) { // ERROR "Proved Leq64$"
return 1337
}
}
return z
}
func mod64uWithIdenticalMax(a, b uint64, ensureBothBranchesCouldHappen bool) int {
a &= 0x10
b &= 0x10 // we need bits.Len64(b.umax) != bits.Len64(b.umax-1)
z := bits.Len64(a % b) // see go.dev/issue/68857 for bits.Len64
if ensureBothBranchesCouldHappen {
if z > bits.Len64(0x10-1) { // ERROR "Disproved Less64$"
return 42
}
} else {
if z <= bits.Len64(0x10-1) { // ERROR "Proved Leq64$"
return 1337
}
}
return z
}
//go:noinline
func useInt(a int) {
}