[dev.typeparams] cmd/compile: simplify SSA devirtualization

This CL implements a few improvements to SSA devirtualization to make
it simpler and more general:

1. Change reflectdata.ITabAddr to now immediately generate the wrapper
functions and write out the itab symbol data. Previously, these were
each handled by separate phases later on.

2. Removes the hack in typecheck where we marked itabs that we
expected to need later. Instead, the calls to ITabAddr in walk now
handle generating the wrappers.

3. Changes the SSA interface call devirtualization algorithm to just
use the itab symbol data (namely, its relocations) to figure out what
pointer is available in memory at the given offset. This decouples it
somewhat from reflectdata.

Change-Id: I8fe06922af8f8a1e7c93f5aff2b60ff59b8e7114
Reviewed-on: https://go-review.googlesource.com/c/go/+/327871
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

2 files changed, 13 insertions, 29 deletions

diff --git a/src/cmd/compile/internal/ssa/config.go b/src/cmd/compile/internal/ssa/config.go
index 61c65f9e54..b08a394368 100644
--- a/src/cmd/compile/internal/ssa/config.go
+++ b/src/cmd/compile/internal/ssa/config.go
@@ -149,12 +149,6 @@ type Frontend interface {
 	// for the parts of that compound type.
 	SplitSlot(parent *LocalSlot, suffix string, offset int64, t *types.Type) LocalSlot
 
-	// DerefItab dereferences an itab function
-	// entry, given the symbol of the itab and
-	// the byte offset of the function pointer.
-	// It may return nil.
-	DerefItab(sym *obj.LSym, offset int64) *obj.LSym
-
 	// Line returns a string describing the given position.
 	Line(src.XPos) string
 
diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 375c4d5a56..115d563933 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -745,27 +745,21 @@ func uaddOvf(a, b int64) bool {
 	return uint64(a)+uint64(b) < uint64(a)
 }
 
-// de-virtualize an InterCall
-// 'sym' is the symbol for the itab
-func devirt(v *Value, aux Aux, sym Sym, offset int64) *AuxCall {
-	f := v.Block.Func
-	n, ok := sym.(*obj.LSym)
-	if !ok {
+// loadLSymOffset simulates reading a word at an offset into a
+// read-only symbol's runtime memory. If it would read a pointer to
+// another symbol, that symbol is returned. Otherwise, it returns nil.
+func loadLSymOffset(lsym *obj.LSym, offset int64) *obj.LSym {
+	if lsym.Type != objabi.SRODATA {
 		return nil
 	}
-	lsym := f.fe.DerefItab(n, offset)
-	if f.pass.debug > 0 {
-		if lsym != nil {
-			f.Warnl(v.Pos, "de-virtualizing call")
-		} else {
-			f.Warnl(v.Pos, "couldn't de-virtualize call")
+
+	for _, r := range lsym.R {
+		if int64(r.Off) == offset && r.Type&^objabi.R_WEAK == objabi.R_ADDR && r.Add == 0 {
+			return r.Sym
 		}
 	}
-	if lsym == nil {
-		return nil
-	}
-	va := aux.(*AuxCall)
-	return StaticAuxCall(lsym, va.abiInfo)
+
+	return nil
 }
 
 // de-virtualize an InterLECall
@@ -776,18 +770,14 @@ func devirtLESym(v *Value, aux Aux, sym Sym, offset int64) *obj.LSym {
 		return nil
 	}
 
-	f := v.Block.Func
-	lsym := f.fe.DerefItab(n, offset)
-	if f.pass.debug > 0 {
+	lsym := loadLSymOffset(n, offset)
+	if f := v.Block.Func; f.pass.debug > 0 {
 		if lsym != nil {
 			f.Warnl(v.Pos, "de-virtualizing call")
 		} else {
 			f.Warnl(v.Pos, "couldn't de-virtualize call")
 		}
 	}
-	if lsym == nil {
-		return nil
-	}
 	return lsym
 }