cmd/compile: check that phis are always first after scheduling

Update #20178

Change-Id: I603f77268ed38afdd84228c775efe006f08f14a7
Reviewed-on: https://go-review.googlesource.com/45018
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -444,6 +444,22 @@ func memCheck(f *Func) {
 			}
 		}
 	}
+
+	// Check that after scheduling, phis are always first in the block.
+	if f.scheduled {
+		for _, b := range f.Blocks {
+			seenNonPhi := false
+			for _, v := range b.Values {
+				if v.Op == OpPhi {
+					if seenNonPhi {
+						f.Fatalf("phi after non-phi @ %s: %s", b, v)
+					}
+				} else {
+					seenNonPhi = true
+				}
+			}
+		}
+	}
 }
 
 // domCheck reports whether x dominates y (including x==y).

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

@@ -117,7 +117,11 @@ func dse(f *Func) {
 		}
 		// walk to previous store
 		if v.Op == OpPhi {
-			continue // At start of block.  Move on to next block.
+			// At start of block.  Move on to next block.
+			// The memory phi, if it exists, is always
+			// the first logical store in the block.
+			// (Even if it isn't the first in the current b.Values order.)
+			continue
 		}
 		for _, a := range v.Args {
 			if a.Block == b && a.Type.IsMemory() {

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

@@ -211,6 +211,8 @@ search:
 		}
 		if v.Op == OpPhi {
 			// A Phi implies we have reached the top of the block.
+			// The memory phi, if it exists, is always
+			// the first logical store in the block.
 			continue search
 		}
 		if v.Type.IsTuple() && v.Type.FieldType(1).IsMemory() {
@@ -228,6 +230,8 @@ search:
 				const limit = 50
 				for i := 0; i < limit; i++ {
 					if m.Op == OpPhi {
+						// The memory phi, if it exists, is always
+						// the first logical store in the block.
 						break
 					}
 					if m.Block.ID != target.Block.ID {