cmd/compile: tweak inlining to favor PPARAM call sites

If a function f being considered for inlining calls
one of its parameters, reduce the normal cost of that
call (57) to 17 to increase the chance that f will
be inlined and (with luck) that parameter will be
revealed as a constant function (which unblocks
escape analysis) or perhaps even be inlined.

The least-change value for that was still effective for
iter_test benchmarks was 32; however tests showed no
particular harm even when reduced as low as 7, and there
have been reports of other performance problems with
rangefunc overheads and so I picked a middling number
in hopes of warding off such reports.

Updates #69015

Change-Id: I2a525c1beffb9f88daa14caa8a622864b023675c
Reviewed-on: https://go-review.googlesource.com/c/go/+/609095
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

3 files changed, 19 insertions, 11 deletions

diff --git a/src/cmd/compile/internal/inline/inl.go b/src/cmd/compile/internal/inline/inl.go
index f343f64952..c38ed8be7f 100644
--- a/src/cmd/compile/internal/inline/inl.go
+++ b/src/cmd/compile/internal/inline/inl.go
@@ -49,6 +49,7 @@ const (
 	inlineExtraAppendCost = 0
 	// default is to inline if there's at most one call. -l=4 overrides this by using 1 instead.
 	inlineExtraCallCost  = 57              // 57 was benchmarked to provided most benefit with no bad surprises; see https://github.com/golang/go/issues/19348#issuecomment-439370742
+	inlineParamCallCost  = 17              // calling a parameter only costs this much extra (inlining might expose a constant function)
 	inlineExtraPanicCost = 1               // do not penalize inlining panics.
 	inlineExtraThrowCost = inlineMaxBudget // with current (2018-05/1.11) code, inlining runtime.throw does not help.
 
@@ -520,6 +521,10 @@ opSwitch:
 			}
 		}
 
+		// A call to a parameter is optimistically a cheap call, if it's a constant function
+		// perhaps it will inline, it also can simplify escape analysis.
+		extraCost := v.extraCallCost
+
 		if n.Fun.Op() == ir.ONAME {
 			name := n.Fun.(*ir.Name)
 			if name.Class == ir.PFUNC {
@@ -539,6 +544,9 @@ opSwitch:
 					}
 				}
 			}
+			if name.Class == ir.PPARAM {
+				extraCost = min(extraCost, inlineParamCallCost)
+			}
 		}
 
 		if cheap {
@@ -572,7 +580,7 @@ opSwitch:
 		}
 
 		// Call cost for non-leaf inlining.
-		v.budget -= v.extraCallCost
+		v.budget -= extraCost
 
 	case ir.OCALLMETH:
 		base.FatalfAt(n.Pos(), "OCALLMETH missed by typecheck")
diff --git a/src/cmd/compile/internal/inline/inlheur/testdata/props/acrosscall.go b/src/cmd/compile/internal/inline/inlheur/testdata/props/acrosscall.go
index a8166fddb6..b9bb87b416 100644
--- a/src/cmd/compile/internal/inline/inlheur/testdata/props/acrosscall.go
+++ b/src/cmd/compile/internal/inline/inlheur/testdata/props/acrosscall.go
@@ -13,7 +13,7 @@ package params
 //   0 ParamFeedsIndirectCall
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[8],"ResultFlags":null}
-// callsite: acrosscall.go:20:12|0 flagstr "" flagval 0 score 60 mask 0 maskstr ""
+// callsite: acrosscall.go:20:12|0 flagstr "" flagval 0 score 20 mask 0 maskstr ""
 // <endcallsites>
 // <endfuncpreamble>
 func T_feeds_indirect_call_via_call_toplevel(f func(int)) {
@@ -25,7 +25,7 @@ func T_feeds_indirect_call_via_call_toplevel(f func(int)) {
 //   0 ParamMayFeedIndirectCall
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[16],"ResultFlags":null}
-// callsite: acrosscall.go:33:13|0 flagstr "" flagval 0 score 60 mask 0 maskstr ""
+// callsite: acrosscall.go:33:13|0 flagstr "" flagval 0 score 20 mask 0 maskstr ""
 // <endcallsites>
 // <endfuncpreamble>
 func T_feeds_indirect_call_via_call_conditional(f func(int)) {
@@ -39,7 +39,7 @@ func T_feeds_indirect_call_via_call_conditional(f func(int)) {
 //   0 ParamMayFeedIndirectCall
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[16],"ResultFlags":null}
-// callsite: acrosscall.go:46:23|0 flagstr "" flagval 0 score 64 mask 0 maskstr ""
+// callsite: acrosscall.go:46:23|0 flagstr "" flagval 0 score 24 mask 0 maskstr ""
 // <endcallsites>
 // <endfuncpreamble>
 func T_feeds_conditional_indirect_call_via_call_toplevel(f func(int)) {
@@ -90,8 +90,8 @@ func T_feeds_conditional_if_via_call(x int) {
 //   1 ParamNoInfo
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[24,0],"ResultFlags":null}
-// callsite: acrosscall.go:98:12|0 flagstr "" flagval 0 score 60 mask 0 maskstr ""
-// callsite: acrosscall.go:99:23|1 flagstr "" flagval 0 score 64 mask 0 maskstr ""
+// callsite: acrosscall.go:98:12|0 flagstr "" flagval 0 score 20 mask 0 maskstr ""
+// callsite: acrosscall.go:99:23|1 flagstr "" flagval 0 score 24 mask 0 maskstr ""
 // <endcallsites>
 // <endfuncpreamble>
 func T_multifeeds1(f1, f2 func(int)) {
diff --git a/src/cmd/compile/internal/inline/inlheur/testdata/props/calls.go b/src/cmd/compile/internal/inline/inlheur/testdata/props/calls.go
index 5cc217b4ba..23dc573f58 100644
--- a/src/cmd/compile/internal/inline/inlheur/testdata/props/calls.go
+++ b/src/cmd/compile/internal/inline/inlheur/testdata/props/calls.go
@@ -133,7 +133,7 @@ func init() {
 // calls.go T_pass_inlinable_func_to_param_feeding_indirect_call 140 0 1
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[0],"ResultFlags":[0]}
-// callsite: calls.go:141:19|0 flagstr "" flagval 0 score 16 mask 512 maskstr "passInlinableFuncToIndCallAdj"
+// callsite: calls.go:141:19|0 flagstr "" flagval 0 score -24 mask 512 maskstr "passInlinableFuncToIndCallAdj"
 // callsite: calls.go:141:19|calls.go:232:10|0 flagstr "" flagval 0 score 2 mask 0 maskstr ""
 // <endcallsites>
 // <endfuncpreamble>
@@ -144,7 +144,7 @@ func T_pass_inlinable_func_to_param_feeding_indirect_call(x int) int {
 // calls.go T_pass_noninlinable_func_to_param_feeding_indirect_call 150 0 1
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[0],"ResultFlags":[0]}
-// callsite: calls.go:153:19|0 flagstr "" flagval 0 score 36 mask 128 maskstr "passFuncToIndCallAdj"
+// callsite: calls.go:153:19|0 flagstr "" flagval 0 score -4 mask 128 maskstr "passFuncToIndCallAdj"
 // <endcallsites>
 // <endfuncpreamble>
 func T_pass_noninlinable_func_to_param_feeding_indirect_call(x int) int {
@@ -158,7 +158,7 @@ func T_pass_noninlinable_func_to_param_feeding_indirect_call(x int) int {
 //   0 ParamFeedsIfOrSwitch
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[32],"ResultFlags":[0]}
-// callsite: calls.go:166:25|0 flagstr "" flagval 0 score 27 mask 1024 maskstr "passInlinableFuncToNestedIndCallAdj"
+// callsite: calls.go:166:25|0 flagstr "" flagval 0 score -13 mask 1024 maskstr "passInlinableFuncToNestedIndCallAdj"
 // callsite: calls.go:166:25|calls.go:237:11|0 flagstr "" flagval 0 score 2 mask 0 maskstr ""
 // <endcallsites>
 // <endfuncpreamble>
@@ -171,7 +171,7 @@ func T_pass_inlinable_func_to_param_feeding_nested_indirect_call(x int) int {
 //   0 ParamFeedsIfOrSwitch
 // <endpropsdump>
 // {"Flags":0,"ParamFlags":[32],"ResultFlags":[0]}
-// callsite: calls.go:178:25|0 flagstr "" flagval 0 score 47 mask 256 maskstr "passFuncToNestedIndCallAdj"
+// callsite: calls.go:178:25|0 flagstr "" flagval 0 score 7 mask 256 maskstr "passFuncToNestedIndCallAdj"
 // <endcallsites>
 // <endfuncpreamble>
 func T_pass_noninlinable_func_to_param_feeding_nested_indirect_call(x int) int {
@@ -183,7 +183,7 @@ func T_pass_noninlinable_func_to_param_feeding_nested_indirect_call(x int) int {
 // {"Flags":0,"ParamFlags":[0,0],"ResultFlags":[0]}
 // callsite: calls.go:209:14|0 flagstr "CallSiteOnPanicPath" flagval 2 score 42 mask 1 maskstr "panicPathAdj"
 // callsite: calls.go:210:15|1 flagstr "CallSiteOnPanicPath" flagval 2 score 42 mask 1 maskstr "panicPathAdj"
-// callsite: calls.go:212:19|2 flagstr "" flagval 0 score 16 mask 512 maskstr "passInlinableFuncToIndCallAdj"
+// callsite: calls.go:212:19|2 flagstr "" flagval 0 score -24 mask 512 maskstr "passInlinableFuncToIndCallAdj"
 // callsite: calls.go:212:19|calls.go:232:10|0 flagstr "" flagval 0 score 4 mask 0 maskstr ""
 // <endcallsites>
 // <endfuncpreamble>