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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>
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@ -49,7 +49,9 @@ func nilcheckelim(f *Func) {
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// value, or a value constructed from an offset of a
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// non-nil ptr (OpAddPtr) implies it is non-nil
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// We also assume unsafe pointer arithmetic generates non-nil pointers. See #27180.
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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 {
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// We assume that SlicePtr is non-nil because we do a bounds check
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// before the slice access (and all cap>0 slices have a non-nil ptr). See #30366.
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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 {
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nonNilValues[v.ID] = true
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}
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}
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@ -61,3 +61,13 @@ func SliceExtensionInt64(s []int, l64 int64) []int {
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// 386:-`.*runtime\.memclr`
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return append(s, make([]int, l64)...)
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}
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// ---------------------- //
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// Nil check of &s[0] //
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// ---------------------- //
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// See issue 30366
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func SliceNilCheck(s []int) {
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p := &s[0]
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// amd64:-`TESTB`
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_ = *p
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}
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