thepudds ed24bb4e60 cmd/compile/internal/escape: propagate constants to interface conversions to avoid allocs ...

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>

2025-05-21 12:02:43 -07:00

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