cmd/5g, cmd/6g, cmd/8g, cmd/9g: use a register to zero in componentgen

Using a zero register results in shorter, faster code.
5g already did this. Bring 6g, 8g, and 9g up to speed.
Reduces godoc binary size by 0.29% using 6g.

This CL includes cosmetic changes to 5g and 8g.
With those cosmetic changes included, componentgen is now
character-for-character equivalent across the four architectures.

Change-Id: I0e13dd48374bad830c725b117a1c86d4197d390c
Reviewed-on: https://go-review.googlesource.com/2606
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/5g/cgen.c

@@ -1626,6 +1626,8 @@ cadable(Node *n)
 /*
  * copy a composite value by moving its individual components.
  * Slices, strings and interfaces are supported.
+ * Small structs or arrays with elements of basic type are
+ * also supported.
  * nr is N when assigning a zero value.
  * return 1 if can do, 0 if cant.
  */
@@ -1700,7 +1702,6 @@ componentgen(Node *nr, Node *nl)
 		freer = 1;
 	}
 
-	
 	// nl and nr are 'cadable' which basically means they are names (variables) now.
 	// If they are the same variable, don't generate any code, because the
 	// VARDEF we generate will mark the old value as dead incorrectly.

src/cmd/6g/cgen.c

@@ -1539,7 +1539,7 @@ cadable(Node *n)
 int
 componentgen(Node *nr, Node *nl)
 {
-	Node nodl, nodr;
+	Node nodl, nodr, tmp;
 	Type *t;
 	int freel, freer;
 	vlong fldcount;
@@ -1599,6 +1599,12 @@ componentgen(Node *nr, Node *nl)
 			igen(nr, &nodr, N);
 			freer = 1;
 		}
+	} else {
+		// When zeroing, prepare a register containing zero.
+		nodconst(&tmp, nl->type, 0);
+		regalloc(&nodr, types[TUINT], N);
+		gmove(&tmp, &nodr);
+		freer = 1;
 	}
 	
 	// nl and nr are 'cadable' which basically means they are names (variables) now.
@@ -1635,8 +1641,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1645,8 +1650,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_cap-Array_nel;
@@ -1655,8 +1659,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_cap-Array_nel;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;
@@ -1670,8 +1673,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1680,8 +1682,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;
@@ -1695,8 +1696,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1705,8 +1705,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;

src/cmd/8g/cgen.c

@@ -1373,13 +1373,15 @@ cadable(Node *n)
 /*
  * copy a composite value by moving its individual components.
  * Slices, strings and interfaces are supported.
+ * Small structs or arrays with elements of basic type are
+ * also supported.
  * nr is N when assigning a zero value.
  * return 1 if can do, 0 if can't.
  */
 int
 componentgen(Node *nr, Node *nl)
 {
-	Node nodl, nodr;
+	Node nodl, nodr, tmp;
 	Type *t;
 	int freel, freer;
 	vlong fldcount;
@@ -1439,6 +1441,12 @@ componentgen(Node *nr, Node *nl)
 			igen(nr, &nodr, N);
 			freer = 1;
 		}
+	} else {
+		// When zeroing, prepare a register containing zero.
+		nodconst(&tmp, nl->type, 0);
+		regalloc(&nodr, types[TUINT], N);
+		gmove(&tmp, &nodr);
+		freer = 1;
 	}
 	
 	// nl and nr are 'cadable' which basically means they are names (variables) now.
@@ -1475,8 +1483,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1485,8 +1492,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_cap-Array_nel;
@@ -1495,8 +1501,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_cap-Array_nel;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;
@@ -1510,8 +1515,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1520,8 +1524,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;
@@ -1535,8 +1538,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1545,8 +1547,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;

src/cmd/9g/cgen.c

@@ -1554,7 +1554,7 @@ cadable(Node *n)
 int
 componentgen(Node *nr, Node *nl)
 {
-	Node nodl, nodr;
+	Node nodl, nodr, tmp;
 	Type *t;
 	int freel, freer;
 	vlong fldcount;
@@ -1614,6 +1614,12 @@ componentgen(Node *nr, Node *nl)
 			igen(nr, &nodr, N);
 			freer = 1;
 		}
+	} else {
+		// When zeroing, prepare a register containing zero.
+		nodconst(&tmp, nl->type, 0);
+		regalloc(&nodr, types[TUINT], N);
+		gmove(&tmp, &nodr);
+		freer = 1;
 	}
 	
 	// nl and nr are 'cadable' which basically means they are names (variables) now.
@@ -1650,8 +1656,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1660,8 +1665,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_cap-Array_nel;
@@ -1670,8 +1674,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_cap-Array_nel;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;
@@ -1685,8 +1688,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1695,8 +1697,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;
@@ -1710,8 +1711,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		nodl.xoffset += Array_nel-Array_array;
@@ -1720,8 +1720,7 @@ componentgen(Node *nr, Node *nl)
 		if(nr != N) {
 			nodr.xoffset += Array_nel-Array_array;
 			nodr.type = nodl.type;
-		} else
+		}
-			nodconst(&nodr, nodl.type, 0);
 		gmove(&nodr, &nodl);
 
 		goto yes;