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cmd/compile: treat slice pointers as non-nil

var a []int = ...
p := &a[0]
_ = *p

We don't need to nil check on the 3rd line. If the bounds check on the 2nd
line passes, we know p is non-nil.

We rely on the fact that any cap>0 slice has a non-nil pointer as its
pointer to the backing array. This is true for all safely-constructed slices,
and I don't see any reason why someone would violate this rule using unsafe.

R=go1.13

Fixes #30366

Change-Id: I3ed764fcb72cfe1fbf963d8c1a82e24e3b6dead7
Reviewed-on: https://go-review.googlesource.com/c/163740
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
This commit is contained in:
Keith Randall 2019-02-25 14:51:58 -08:00 committed by Keith Randall
parent 57976fe2b4
commit 933e34ac99
2 changed files with 13 additions and 1 deletions

View File

@ -49,7 +49,9 @@ func nilcheckelim(f *Func) {
// value, or a value constructed from an offset of a
// non-nil ptr (OpAddPtr) implies it is non-nil
// We also assume unsafe pointer arithmetic generates non-nil pointers. See #27180.
if v.Op == OpAddr || v.Op == OpLocalAddr || v.Op == OpAddPtr || v.Op == OpOffPtr || v.Op == OpAdd32 || v.Op == OpAdd64 || v.Op == OpSub32 || v.Op == OpSub64 {
// We assume that SlicePtr is non-nil because we do a bounds check
// before the slice access (and all cap>0 slices have a non-nil ptr). See #30366.
if v.Op == OpAddr || v.Op == OpLocalAddr || v.Op == OpAddPtr || v.Op == OpOffPtr || v.Op == OpAdd32 || v.Op == OpAdd64 || v.Op == OpSub32 || v.Op == OpSub64 || v.Op == OpSlicePtr {
nonNilValues[v.ID] = true
}
}

View File

@ -61,3 +61,13 @@ func SliceExtensionInt64(s []int, l64 int64) []int {
// 386:-`.*runtime\.memclr`
return append(s, make([]int, l64)...)
}
// ---------------------- //
// Nil check of &s[0] //
// ---------------------- //
// See issue 30366
func SliceNilCheck(s []int) {
p := &s[0]
// amd64:-`TESTB`
_ = *p
}