cmd/compile: compute Divu's limits from argument's limits

Change-Id: Id522bde5bba627d9cdc8c3d8e907bdc168e5b13c
Reviewed-on: https://go-review.googlesource.com/c/go/+/605157
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

diff --git a/src/cmd/compile/internal/ssa/prove.go b/src/cmd/compile/internal/ssa/prove.go
index c90b3800969078fda6a6c17396eb6b116892c5a3..40948576143edcdad5a438cf97f681612f175e40 100644 (file)
--- a/src/cmd/compile/internal/ssa/prove.go
+++ b/src/cmd/compile/internal/ssa/prove.go
@@ -1780,6 +1780,17 @@ func (ft *factsTable) flowLimit(v *Value) bool {
                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 OpDiv64u, OpDiv32u, OpDiv16u, OpDiv8u:
+               a := ft.limits[v.Args[0].ID]
+               b := ft.limits[v.Args[1].ID]
+               lim := noLimit
+               if b.umax > 0 {
+                       lim = lim.unsignedMin(a.umin / b.umax)
+               }
+               if b.umin > 0 {
+                       lim = lim.unsignedMax(a.umax / b.umin)
+               }
+               return ft.newLimit(v, lim)
 
        case OpPhi:
                // Compute the union of all the input phis.

diff --git a/test/prove.go b/test/prove.go
index a1aa67d4729e7d41e133b51db7dc658388f61391..5ea81bffff45b81e3c8c1e88f948645012054eef 100644 (file)
--- a/test/prove.go
+++ b/test/prove.go
@@ -1478,6 +1478,29 @@ func mod64uWithIdenticalMax(a, b uint64, ensureBothBranchesCouldHappen bool) int
        return z
 }
 
+func div64u(a, b uint64, ensureAllBranchesCouldHappen func() bool) uint64 {
+       a &= 0xffff
+       a |= 0xfff
+       b &= 0xff
+       b |= 0xf
+
+       z := a / b // ERROR "Proved Neq64$"
+
+       if ensureAllBranchesCouldHappen() && z > 0xffff/0xf { // ERROR "Disproved Less64U$"
+               return 42
+       }
+       if ensureAllBranchesCouldHappen() && z <= 0xffff/0xf { // ERROR "Proved Leq64U$"
+               return 1337
+       }
+       if ensureAllBranchesCouldHappen() && z < 0xfff/0xff { // ERROR "Disproved Less64U$"
+               return 42
+       }
+       if ensureAllBranchesCouldHappen() && z >= 0xfff/0xff { // ERROR "Proved Leq64U$"
+               return 42
+       }
+       return z
+}
+
 //go:noinline
 func useInt(a int) {
 }