[dev.regabi] cmd/compile: earlier deadcode removal

This CL moves the general deadcode-removal pass to before computing
Addrtaken, which allows variables to still be converted to SSA if
their address is only taken in unreachable code paths (e.g., the "&mp"
expression in the "if false" block in runtime/os_linux.go:newosproc).

This doesn't pass toolstash -cmp, because it allows SSA to better
optimize some code.

Change-Id: I43e54acc02fdcbad8eb6493283f355aa1ee0de84
Reviewed-on: https://go-review.googlesource.com/c/go/+/280992
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

1 files changed, 8 insertions, 8 deletions

diff --git a/src/cmd/compile/internal/gc/main.go b/src/cmd/compile/internal/gc/main.go
index c1f51e4f1d..2ea614e17f 100644
--- a/src/cmd/compile/internal/gc/main.go
+++ b/src/cmd/compile/internal/gc/main.go
@@ -213,6 +213,14 @@ func Main(archInit func(*ssagen.ArchInfo)) {
 		typecheck.Export(initTask)
 	}
 
+	// Eliminate some obviously dead code.
+	// Must happen after typechecking.
+	for _, n := range typecheck.Target.Decls {
+		if n.Op() == ir.ODCLFUNC {
+			deadcode.Func(n.(*ir.Func))
+		}
+	}
+
 	// Compute Addrtaken for names.
 	// We need to wait until typechecking is done so that when we see &x[i]
 	// we know that x has its address taken if x is an array, but not if x is a slice.
@@ -224,14 +232,6 @@ func Main(archInit func(*ssagen.ArchInfo)) {
 	}
 	typecheck.IncrementalAddrtaken = true
 
-	// Eliminate some obviously dead code.
-	// Must happen after typechecking.
-	for _, n := range typecheck.Target.Decls {
-		if n.Op() == ir.ODCLFUNC {
-			deadcode.Func(n.(*ir.Func))
-		}
-	}
-
 	// Decide how to capture closed variables.
 	// This needs to run before escape analysis,
 	// because variables captured by value do not escape.