cmd/compile: remove dead values after flagalloc

Fix a longstanding TODO.

Provides widespread, minor improvements.
Negligible compiler cost.

Because the freeze nears, put in a safety flag to easily disable.

Change-Id: I338812181ab6d806fecf22afd3c3502e2c94f7a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/229600
Run-TryBot: Josh Bleecher Snyder <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

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

@@ -106,7 +106,7 @@ func flagalloc(f *Func) {
 	}
 
 	// Add flag spill and recomputation where they are needed.
-	// TODO: Remove original instructions if they are never used.
+	var remove []*Value // values that should be checked for possible removal
 	var oldSched []*Value
 	for _, b := range f.Blocks {
 		oldSched = append(oldSched[:0], b.Values...)
@@ -131,6 +131,7 @@ func flagalloc(f *Func) {
 			// If v will be spilled, and v uses memory, then we must split it
 			// into a load + a flag generator.
 			if spill[v.ID] && v.MemoryArg() != nil {
+				remove = append(remove, v)
 				if !f.Config.splitLoad(v) {
 					f.Fatalf("can't split flag generator: %s", v.LongString())
 				}
@@ -164,6 +165,7 @@ func flagalloc(f *Func) {
 		for i, v := range b.ControlValues() {
 			if v != flag && v.Type.IsFlags() {
 				// Recalculate control value.
+				remove = append(remove, v)
 				c := copyFlags(v, b)
 				b.ReplaceControl(i, c)
 				flag = v
@@ -172,19 +174,74 @@ func flagalloc(f *Func) {
 		if v := end[b.ID]; v != nil && v != flag {
 			// Need to reissue flag generator for use by
 			// subsequent blocks.
+			remove = append(remove, v)
 			copyFlags(v, b)
 			// Note: this flag generator is not properly linked up
 			// with the flag users. This breaks the SSA representation.
 			// We could fix up the users with another pass, but for now
-			// we'll just leave it.  (Regalloc has the same issue for
+			// we'll just leave it. (Regalloc has the same issue for
 			// standard regs, and it runs next.)
+			// For this reason, take care not to add this flag
+			// generator to the remove list.
 		}
 	}
 
 	// Save live flag state for later.
 	for _, b := range f.Blocks {
 		b.FlagsLiveAtEnd = end[b.ID] != nil
 	}
+
+	const go115flagallocdeadcode = true
+	if !go115flagallocdeadcode {
+		return
+	}
+
+	// Remove any now-dead values.
+	// The number of values to remove is likely small,
+	// and removing them requires processing all values in a block,
+	// so minimize the number of blocks that we touch.
+
+	// Shrink remove to contain only dead values, and clobber those dead values.
+	for i := 0; i < len(remove); i++ {
+		v := remove[i]
+		if v.Uses == 0 {
+			v.reset(OpInvalid)
+			continue
+		}
+		// Remove v.
+		last := len(remove) - 1
+		remove[i] = remove[last]
+		remove[last] = nil
+		remove = remove[:last]
+		i-- // reprocess value at i
+	}
+
+	if len(remove) == 0 {
+		return
+	}
+
+	removeBlocks := f.newSparseSet(f.NumBlocks())
+	defer f.retSparseSet(removeBlocks)
+	for _, v := range remove {
+		removeBlocks.add(v.Block.ID)
+	}
+
+	// Process affected blocks, preserving value order.
+	for _, b := range f.Blocks {
+		if !removeBlocks.contains(b.ID) {
+			continue
+		}
+		i := 0
+		for j := 0; j < len(b.Values); j++ {
+			v := b.Values[j]
+			if v.Op == OpInvalid {
+				continue
+			}
+			b.Values[i] = v
+			i++
+		}
+		b.truncateValues(i)
+	}
 }
 
 func (v *Value) clobbersFlags() bool {