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Currently, the integer value in the following interface conversion gets heap allocated: v := 1000 fmt.Println(v) In contrast, this conversion does not currently cause the integer value to be heap allocated: fmt.Println(1000) The second example is able to avoid heap allocation because of an optimization in walk (by Josh in #18704 and related issues) that recognizes a literal is being used. In the first example, that optimization is currently thwarted by the literal getting assigned to a local variable prior to use in the interface conversion. This CL propagates constants to interface conversions like in the first example to avoid heap allocations, instead using a read-only global. The net effect is roughly turning the first example into the second. One place this comes up in practice currently is with logging or debug prints. For example, if we have something like: func conditionalDebugf(format string, args ...interface{}) { if debugEnabled { fmt.Fprintf(io.Discard, format, args...) } } Prior to this CL, this integer is heap allocated, even when the debugEnabled flag is false, and even when the compiler inlines conditionalDebugf: v := 1000 conditionalDebugf("hello %d", v) With this CL, the integer here is no longer heap allocated, even when the debugEnabled flag is enabled, because the compiler can now see that it can use a read-only global. See the writeup in #71359 for more details. CL 649076 (earlier in our stack) added most of the tests along with debug diagnostics in convert.go to make it easier to test this change. Updates #71359 Updates #62653 Updates #53465 Updates #8618 Change-Id: I19a51e74b36576ebb0b9cf599267cbd2bd847ce4 Reviewed-on: https://go-review.googlesource.com/c/go/+/649079 Auto-Submit: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@golang.org> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Keith Randall <khr@google.com>
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