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Bounds Check Elimination

Go is a memory safe language. In array/slice element indexing and subslice operations, Go runtime will check whether or not the involved indexes are out of range. If an index is out of range, a panic will be produced to prevent the invalid index from doing harm. This is called bounds check. Bounds checks make our code run safely, on the other hand, they also make our code run a little slower.

Since Go Toolchain 1.7, the standard Go compiler has used a new compiler backend, which based on SSA (static single-assignment form). SSA helps Go compilers effectively use optimizations like BCE (bounds check elimination) and CSE (common subexpression elimination). BCE can avoid some unnecessary bounds checks, and CSE can avoid some duplicate calculations, so that the standard Go compiler can generate more efficient programs. Sometimes the improvement effects of these optimizations are obvious.

This article will list some examples to show how BCE works with the standard Go compiler 1.7+.

For Go Toolchain 1.7+, we can run go build -gcflags="-d=ssa/check_bce/debug=1" to show which code lines still need bounds checks.

Example 1

// example1.go package main func f1(s []int) { _ = s[0] // line 5: bounds check _ = s[1] // line 6: bounds check _ = s[2] // line 7: bounds check } func f2(s []int) { _ = s[2] // line 11: bounds check _ = s[1] // line 12: bounds check eliminated! _ = s[0] // line 13: bounds check eliminated! } func f3(s []int, index int) { _ = s[index] // line 17: bounds check _ = s[index] // line 18: bounds check eliminated! } func f4(a [5]int) { _ = a[4] // line 22: bounds check eliminated! } func main() {} $ go build -gcflags="-d=ssa/check_bce/debug=1" example1.go ./example1.go:5: Found IsInBounds ./example1.go:6: Found IsInBounds ./example1.go:7: Found IsInBounds ./example1.go:11: Found IsInBounds ./example1.go:17: Found IsInBounds We can see that there are no needs to do bounds checks for line 12 and line 13 in function f2 , for the bounds check at line 11 ensures that the indexes in line 12 and line 13 will not be out of range. But in function f1 , bounds checks must be performed for all three lines. The bounds check at line 5 can't ensure line 6 and line 7 are safe, and the bounds check at line 6 can't ensure line 7 is safe. For function f3 , the compiler knows the second s[index] is absolutely safe if the first s[index] is safe. The compiler also correctly thinks the only line (line 22) in function f4 is safe.

Example 2

// example2.go package main func f5(s []int) { for i := range s { _ = s[i] _ = s[i:len(s)] _ = s[:i+1] } } func f6(s []int) { for i := 0; i < len(s); i++ { _ = s[i] _ = s[i:len(s)] _ = s[:i+1] } } func f7(s []int) { for i := len(s) - 1; i >= 0; i-- { _ = s[i] _ = s[i:len(s)] } } func f8(s []int, index int) { if index >= 0 && index < len(s) { _ = s[index] _ = s[index:len(s)] } } func f9(s []int) { if len(s) > 2 { _, _, _ = s[0], s[1], s[2] } } func main() {} $ go build -gcflags="-d=ssa/check_bce/debug=1" example2.go Cool! The standard compiler removes all bound checks in this program. Note: before Go Toolchain version 1.11, the standard compiler is not smart enough to detect line 22 is safe.

Example 3

// example3.go package main import "math/rand" func fa() { s := []int{0, 1, 2, 3, 4, 5, 6} index := rand.Intn(7) _ = s[:index] // line 9: bounds check _ = s[index:] // line 10: bounds check eliminated! } func fb(s []int, i int) { _ = s[:i] // line 14: bounds check _ = s[i:] // line 15: bounds check, not smart enough? } func fc() { s := []int{0, 1, 2, 3, 4, 5, 6} s = s[:4] i := rand.Intn(7) _ = s[:i] // line 22: bounds check _ = s[i:] // line 23: bounds check, not smart enough? } func main() {} $ go build -gcflags="-d=ssa/check_bce/debug=1" example3.go ./example3.go:9: Found IsSliceInBounds ./example3.go:14: Found IsSliceInBounds ./example3.go:15: Found IsSliceInBounds ./example3.go:22: Found IsSliceInBounds ./example3.go:23: Found IsSliceInBounds Oh, so many places still need to do bounds check! But wait, why does the standard Go compiler think line 10 is safe but line 15 and line 23 are not? Is the compiler still not smart enough? In fact, the compiler is right here! Why? The reason is the start index in a subslice expression may be larger than the length of the base slice. Let's view a simple example: package main func main() { s0 := make([]int, 5, 10) // len(s0) == 5, cap(s0) == 10 index := 8 // In Go, for the subslice syntax s[a:b], // the relations 0 <= a <= b <= cap(s) must // be ensured to avoid panicking. _ = s0[:index] // The above line is safe can't ensure the // following line is also safe. In fact, the // following line will panic, for the starting // index is larger than the end index. _ = s0[index:] // panic } So the conclusion that if s[:index] is safe then s[index:] is also safe is only right when len(s) is equal to cap(s) . This is why the code lines in function fb and fc of example 3 still need to do bounds checks. Standard Go compiler successfully detects len(s) is equal to cap(s) in function fa . Great work! Go team!

Example 4

// example4.go package main import "math/rand" func fb2(s []int, index int) { _ = s[index:] // line 7: bounds check _ = s[:index] // line 8: bounds check eliminated! } func fc2() { s := []int{0, 1, 2, 3, 4, 5, 6} s = s[:4] index := rand.Intn(7) _ = s[index:] // line 15 bounds check _ = s[:index] // line 16: bounds check eliminated! } func main() {} $ go build -gcflags="-d=ssa/check_bce/debug=1" example4.go ./example4.go:7:7: Found IsSliceInBounds ./example4.go:15:7: Found IsSliceInBounds In this example, The standard Go compiler successfully concludes line 8 is also safe if line 7 is safe in function fb2 .

is also safe if line is safe in function . line 16 is also safe if line 15 is safe in function fc2 . In this example, The standard Go compiler successfully concludes Note: the standard Go compiler in Go Toolchain earlier than version 1.9 fails to detect line 8 doesn't need bounds check.

Example 5

// example5.go package main func fd(is []int, bs []byte) { if len(is) >= 256 { for _, n := range bs { _ = is[n] // line 7: bounds check } } } func fd2(is []int, bs []byte) { if len(is) >= 256 { is = is[:256] // line 14: a hint for _, n := range bs { _ = is[n] // line 16: BCEed! } } } func fe(isa []int, isb []int) { if len(isa) > 0xFFF { for _, n := range isb { _ = isa[n & 0xFFF] // line 24: bounds check } } } func fe2(isa []int, isb []int) { if len(isa) > 0xFFF { isa = isa[:0xFFF+1] // line 31: a hint for _, n := range isb { _ = isa[n & 0xFFF] // line 33: BCEed! } } } func main() {} $ go build -gcflags="-d=ssa/check_bce/debug=1" example5.go ./example5.go:7: Found IsInBounds ./example5.go:24: Found IsInBounds The current version of the standard Go compiler is not smart enough to eliminate all unnecessary bounds checks. Sometimes, we can make some hints to help the compiler eliminate some unnecessary bounds checks.

Summary

There are more BCE optimizations made by the standard Go compiler. They might be not as abvious as the above listed ones, So this article will not show them all.

Although the BCE feature in the standard Go compiler is still not perfect, it really does well for many common cases. It is no doubt that standard Go compiler will do better in later versions so that it is possible the hints made in the above 5th example will become unnecessary. Thank Go team for adding this wonderful feature!

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