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.

This is a re-submit of PR#59575, which fix a broken dominance relationship caught by ssacheck

Updates #59399

Change-Id: I57482dee38f8e80a610aed4f64295e60c38b7a47
GitHub-Last-Rev: 830016f
GitHub-Pull-Request: #60469
Reviewed-on: https://go-review.googlesource.com/c/go/+/498795
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: y1yang0

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

@@ -649,6 +649,8 @@ 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},