From 933e34ac995cf7b2a1d044dc530bfafbf697758d Mon Sep 17 00:00:00 2001 From: Keith Randall Date: Mon, 25 Feb 2019 14:51:58 -0800 Subject: [PATCH] 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 Reviewed-by: Josh Bleecher Snyder --- src/cmd/compile/internal/ssa/nilcheck.go | 4 +++- test/codegen/slices.go | 10 ++++++++++ 2 files changed, 13 insertions(+), 1 deletion(-) diff --git a/src/cmd/compile/internal/ssa/nilcheck.go b/src/cmd/compile/internal/ssa/nilcheck.go index 5f58e2d7ec1..5369a51023c 100644 --- a/src/cmd/compile/internal/ssa/nilcheck.go +++ b/src/cmd/compile/internal/ssa/nilcheck.go @@ -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 } } diff --git a/test/codegen/slices.go b/test/codegen/slices.go index 15dbcee7372..6477c6f6c78 100644 --- a/test/codegen/slices.go +++ b/test/codegen/slices.go @@ -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 +}