cmd/compile: sparse conditional constant propagation

sparse conditional constant propagation can discover optimization opportunities that cannot be found by just combining constant folding and constant propagation and dead code elimination separately.

Updates #59399

Change-Id: Ia954e906480654a6f0cc065d75b5912f96f36b2e
GitHub-Last-Rev: 90fc02d
GitHub-Pull-Request: #59575
Reviewed-on: https://go-review.googlesource.com/c/go/+/483875
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Run-TryBot: Keith Randall <[email protected]>

Author: y1yang0

src/cmd/compile/internal/ssa/_gen/genericOps.go

@@ -643,6 +643,8 @@ var genericOps = []opData{
 //    Plain                []            [next]
 //       If   [boolean Value]      [then, else]
 //    First                []   [always, never]
+//    Defer             [mem]  [nopanic, panic]                  (control opcode should be OpStaticCall to runtime.deferproc)
+//JumpTable   [integer Value]  [succ1,succ2,..]
 
 var genericBlocks = []blockData{
 	{name: "Plain"},                  // a single successor

src/cmd/compile/internal/ssa/block.go

@@ -112,13 +112,6 @@ func (e Edge) String() string {
 }
 
 // BlockKind is the kind of SSA block.
-//
-//	  kind          controls        successors
-//	------------------------------------------
-//	  Exit      [return mem]                []
-//	 Plain                []            [next]
-//	    If   [boolean Value]      [then, else]
-//	 Defer             [mem]  [nopanic, panic]  (control opcode should be OpStaticCall to runtime.deferproc)
 type BlockKind int16
 
 // short form print
@@ -275,8 +268,7 @@ func (b *Block) truncateValues(i int) {
 	b.Values = b.Values[:i]
 }
 
-// AddEdgeTo adds an edge from block b to block c. Used during building of the
-// SSA graph; do not use on an already-completed SSA graph.
+// AddEdgeTo adds an edge from block b to block c.
 func (b *Block) AddEdgeTo(c *Block) {
 	i := len(b.Succs)
 	j := len(c.Preds)

src/cmd/compile/internal/ssa/compile.go

@@ -477,6 +477,7 @@ var passes = [...]pass{
 	{name: "softfloat", fn: softfloat, required: true},
 	{name: "late opt", fn: opt, required: true}, // TODO: split required rules and optimizing rules
 	{name: "dead auto elim", fn: elimDeadAutosGeneric},
+	{name: "sccp", fn: sccp},
 	{name: "generic deadcode", fn: deadcode, required: true}, // remove dead stores, which otherwise mess up store chain
 	{name: "check bce", fn: checkbce},
 	{name: "branchelim", fn: branchelim},