cmd/compile: remove debugging option InlineSCCOnePass from inliner

Delete the "InlineSCCOnePass" debugging flag and the inliner fallback
code that kicks in if it is used. The change it was intended to guard
has been working on tip for some time, no need for the fallback any
more.

Updates #58905.

Change-Id: I2e1dbc7640902d9402213db5ad338be03deb96c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/492015
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>
Run-TryBot: Than McIntosh <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/cmd/compile/internal/base/debug.go

@@ -32,7 +32,6 @@ type DebugFlags struct {
 	InlFuncsWithClosures  int    `help:"allow functions with closures to be inlined" concurrent:"ok"`
 	InlStaticInit         int    `help:"allow static initialization of inlined calls" concurrent:"ok"`
 	InterfaceCycles       int    `help:"allow anonymous interface cycles"`
-	InlineSCCOnePass      int    `help:"visit SCC funcs only once during inlining (legacy behavior)"`
 	Libfuzzer             int    `help:"enable coverage instrumentation for libfuzzer"`
 	LoopVar               int    `help:"shared (0, default), 1 (private loop variables), 2, private + log"`
 	LoopVarHash           string `help:"for debugging changes in loop behavior. Overrides experiment and loopvar flag."`

src/cmd/compile/internal/inline/inl.go

@@ -211,25 +211,15 @@ func InlineDecls(p *pgo.Profile, decls []ir.Node, doInline bool) {
 		// before performing any inlining, the results are less
 		// sensitive to the order within the SCC (see #58905 for an
 		// example).
-		if base.Debug.InlineSCCOnePass == 0 {
-			// Compute inlinability for all functions in the SCC ...
-			for _, n := range list {
-				doCanInline(n, recursive, numfns)
-			}
-			// ... then make a second pass to do inlining of calls.
-			if doInline {
-				for _, n := range list {
-					InlineCalls(n, p)
-				}
-			}
-		} else {
-			// Legacy ordering to make it easier to triage any bugs
-			// or compile time issues that might crop up.
+
+		// First compute inlinability for all functions in the SCC ...
+		for _, n := range list {
+			doCanInline(n, recursive, numfns)
+		}
+		// ... then make a second pass to do inlining of calls.
+		if doInline {
 			for _, n := range list {
-				doCanInline(n, recursive, numfns)
-				if doInline {
-					InlineCalls(n, p)
-				}
+				InlineCalls(n, p)
 			}
 		}
 	})