diff --git a/src/cmd/compile/internal/ssa/cse.go b/src/cmd/compile/internal/ssa/cse.go
index 25f424fbee..58c52f23e6 100644
--- a/src/cmd/compile/internal/ssa/cse.go
+++ b/src/cmd/compile/internal/ssa/cse.go
@@ -153,7 +153,6 @@ func cse(f *Func) {
 					i++
 				}
 			}
-			// TODO(khr): if value is a control value, do we need to keep it block-local?
 		}
 	}
 
@@ -166,6 +165,16 @@ func cse(f *Func) {
 				}
 			}
 		}
+		if v := b.Control; v != nil {
+			if x := rewrite[v.ID]; x != nil {
+				if v.Op == OpNilCheck {
+					// nilcheck pass will remove the nil checks and log
+					// them appropriately, so don't mess with them here.
+					continue
+				}
+				b.Control = x
+			}
+		}
 	}
 }
 
diff --git a/src/cmd/compile/internal/ssa/flagalloc.go b/src/cmd/compile/internal/ssa/flagalloc.go
index c088158057..714ac016a2 100644
--- a/src/cmd/compile/internal/ssa/flagalloc.go
+++ b/src/cmd/compile/internal/ssa/flagalloc.go
@@ -21,6 +21,15 @@ func flagalloc(f *Func) {
 		// Walk blocks backwards.  Poor-man's postorder traversal.
 		for i := len(f.Blocks) - 1; i >= 0; i-- {
 			b := f.Blocks[i]
+			if len(b.Preds) > 1 {
+				// Don't use any flags register at the start
+				// of a merge block.  This causes problems
+				// in regalloc because some of the rematerialization
+				// instructions used on incoming merge edges clobber
+				// the flags register.
+				// TODO: only for architectures where this matters?
+				continue
+			}
 			// Walk values backwards to figure out what flag
 			// value we want in the flag register at the start
 			// of the block.
diff --git a/src/cmd/compile/internal/ssa/gen/AMD64.rules b/src/cmd/compile/internal/ssa/gen/AMD64.rules
index 7d0aa4b2d3..0edbfdaa1a 100644
--- a/src/cmd/compile/internal/ssa/gen/AMD64.rules
+++ b/src/cmd/compile/internal/ssa/gen/AMD64.rules
@@ -370,7 +370,7 @@
 (If (SETGF  cmp) yes no) -> (UGT  cmp yes no)
 (If (SETGEF cmp) yes no) -> (UGE  cmp yes no)
 (If (SETEQF cmp) yes no) -> (EQF  cmp yes no)
-(If (SETNEF cmp) yes no) -> (EQF  cmp yes no)
+(If (SETNEF cmp) yes no) -> (NEF  cmp yes no)
 
 (If cond yes no) -> (NE (TESTB cond cond) yes no)
 
diff --git a/src/cmd/compile/internal/ssa/gen/AMD64Ops.go b/src/cmd/compile/internal/ssa/gen/AMD64Ops.go
index ba53e81ddd..461026bd7b 100644
--- a/src/cmd/compile/internal/ssa/gen/AMD64Ops.go
+++ b/src/cmd/compile/internal/ssa/gen/AMD64Ops.go
@@ -433,7 +433,7 @@ func init() {
 			name: "DUFFCOPY",
 			reg: regInfo{
 				inputs:   []regMask{buildReg("DI"), buildReg("SI")},
-				clobbers: buildReg("DI SI X0"), // uses X0 as a temporary
+				clobbers: buildReg("DI SI X0 FLAGS"), // uses X0 as a temporary
 			},
 		},
 
diff --git a/src/cmd/compile/internal/ssa/opGen.go b/src/cmd/compile/internal/ssa/opGen.go
index 132ca83f95..bbedf2fb64 100644
--- a/src/cmd/compile/internal/ssa/opGen.go
+++ b/src/cmd/compile/internal/ssa/opGen.go
@@ -3177,7 +3177,7 @@ var opcodeTable = [...]opInfo{
 				{0, 128}, // .DI
 				{1, 64},  // .SI
 			},
-			clobbers: 65728, // .SI .DI .X0
+			clobbers: 8590000320, // .SI .DI .X0 .FLAGS
 		},
 	},
 	{
diff --git a/src/cmd/compile/internal/ssa/rewriteAMD64.go b/src/cmd/compile/internal/ssa/rewriteAMD64.go
index 3be94e37e7..5c2f3db4b2 100644
--- a/src/cmd/compile/internal/ssa/rewriteAMD64.go
+++ b/src/cmd/compile/internal/ssa/rewriteAMD64.go
@@ -14213,23 +14213,23 @@ func rewriteBlockAMD64(b *Block) bool {
 		;
 		// match: (If (SETNEF cmp) yes no)
 		// cond:
-		// result: (EQF  cmp yes no)
+		// result: (NEF  cmp yes no)
 		{
 			v := b.Control
 			if v.Op != OpAMD64SETNEF {
-				goto endfe25939ca97349543bc2d2ce4f97ba41
+				goto endaa989df10b5bbc5fdf8f7f0b81767e86
 			}
 			cmp := v.Args[0]
 			yes := b.Succs[0]
 			no := b.Succs[1]
-			b.Kind = BlockAMD64EQF
+			b.Kind = BlockAMD64NEF
 			b.Control = cmp
 			b.Succs[0] = yes
 			b.Succs[1] = no
 			return true
 		}
-		goto endfe25939ca97349543bc2d2ce4f97ba41
-	endfe25939ca97349543bc2d2ce4f97ba41:
+		goto endaa989df10b5bbc5fdf8f7f0b81767e86
+	endaa989df10b5bbc5fdf8f7f0b81767e86:
 		;
 		// match: (If cond yes no)
 		// cond: