objtool: Make instruction::alts a single-linked list

 struct instruction {
 	struct list_head           list;                 /*     0    16 */
 	struct hlist_node          hash;                 /*    16    16 */
 	struct list_head           call_node;            /*    32    16 */
 	struct section *           sec;                  /*    48     8 */
 	long unsigned int          offset;               /*    56     8 */
 	/* --- cacheline 1 boundary (64 bytes) --- */
 	unsigned int               len;                  /*    64     4 */
 	enum insn_type             type;                 /*    68     4 */
 	long unsigned int          immediate;            /*    72     8 */
 	u16                        dead_end:1;           /*    80: 0  2 */
 	u16                        ignore:1;             /*    80: 1  2 */
 	u16                        ignore_alts:1;        /*    80: 2  2 */
 	u16                        hint:1;               /*    80: 3  2 */
 	u16                        save:1;               /*    80: 4  2 */
 	u16                        restore:1;            /*    80: 5  2 */
 	u16                        retpoline_safe:1;     /*    80: 6  2 */
 	u16                        noendbr:1;            /*    80: 7  2 */
 	u16                        entry:1;              /*    80: 8  2 */

 	/* XXX 7 bits hole, try to pack */

 	s8                         instr;                /*    82     1 */
 	u8                         visited;              /*    83     1 */

 	/* XXX 4 bytes hole, try to pack */

 	struct alt_group *         alt_group;            /*    88     8 */
 	struct symbol *            call_dest;            /*    96     8 */
 	struct instruction *       jump_dest;            /*   104     8 */
 	struct instruction *       first_jump_src;       /*   112     8 */
 	struct reloc *             jump_table;           /*   120     8 */
 	/* --- cacheline 2 boundary (128 bytes) --- */
 	struct reloc *             reloc;                /*   128     8 */
-	struct list_head           alts;                 /*   136    16 */
-	struct symbol *            sym;                  /*   152     8 */
-	struct stack_op *          stack_ops;            /*   160     8 */
-	struct cfi_state *         cfi;                  /*   168     8 */
+	struct alternative *       alts;                 /*   136     8 */
+	struct symbol *            sym;                  /*   144     8 */
+	struct stack_op *          stack_ops;            /*   152     8 */
+	struct cfi_state *         cfi;                  /*   160     8 */

-	/* size: 176, cachelines: 3, members: 29 */
-	/* sum members: 170, holes: 1, sum holes: 4 */
+	/* size: 168, cachelines: 3, members: 29 */
+	/* sum members: 162, holes: 1, sum holes: 4 */
 	/* sum bitfield members: 9 bits, bit holes: 1, sum bit holes: 7 bits */
-	/* last cacheline: 48 bytes */
+	/* last cacheline: 40 bytes */
 };

pre:	5:58.50 real,   229.64 user,    128.65 sys,     26221520 mem
post:	5:48.86 real,   220.30 user,    128.34 sys,     24834672 mem

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org> # build only
Tested-by: Thomas Weißschuh <linux@weissschuh.net> # compile and run
Link: https://lore.kernel.org/r/20230208172245.430556498@infradead.org

Author: Peter Zijlstra

tools/objtool/check.c

@@ -23,7 +23,7 @@
 #include <linux/static_call_types.h>
 
 struct alternative {
-	struct list_head list;
+	struct alternative *next;
 	struct instruction *insn;
 	bool skip_orig;
 };
@@ -397,7 +397,6 @@ static int decode_instructions(struct objtool_file *file)
 				return -1;
 			}
 			memset(insn, 0, sizeof(*insn));
-			INIT_LIST_HEAD(&insn->alts);
 			INIT_LIST_HEAD(&insn->call_node);
 
 			insn->sec = sec;
@@ -1780,7 +1779,6 @@ static int handle_group_alt(struct objtool_file *file,
 			return -1;
 		}
 		memset(nop, 0, sizeof(*nop));
-		INIT_LIST_HEAD(&nop->alts);
 
 		nop->sec = special_alt->new_sec;
 		nop->offset = special_alt->new_off + special_alt->new_len;
@@ -1978,7 +1976,8 @@ static int add_special_section_alts(struct objtool_file *file)
 		alt->insn = new_insn;
 		alt->skip_orig = special_alt->skip_orig;
 		orig_insn->ignore_alts |= special_alt->skip_alt;
-		list_add_tail(&alt->list, &orig_insn->alts);
+		alt->next = orig_insn->alts;
+		orig_insn->alts = alt;
 
 		list_del(&special_alt->list);
 		free(special_alt);
@@ -2037,7 +2036,8 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn,
 		}
 
 		alt->insn = dest_insn;
-		list_add_tail(&alt->list, &insn->alts);
+		alt->next = insn->alts;
+		insn->alts = alt;
 		prev_offset = reloc->offset;
 	}
 
@@ -3594,10 +3594,10 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 		if (propagate_alt_cfi(file, insn))
 			return 1;
 
-		if (!insn->ignore_alts && !list_empty(&insn->alts)) {
+		if (!insn->ignore_alts && insn->alts) {
 			bool skip_orig = false;
 
-			list_for_each_entry(alt, &insn->alts, list) {
+			for (alt = insn->alts; alt; alt = alt->next) {
 				if (alt->skip_orig)
 					skip_orig = true;
 
@@ -3796,11 +3796,11 @@ static int validate_entry(struct objtool_file *file, struct instruction *insn)
 
 		insn->visited |= VISITED_ENTRY;
 
-		if (!insn->ignore_alts && !list_empty(&insn->alts)) {
+		if (!insn->ignore_alts && insn->alts) {
 			struct alternative *alt;
 			bool skip_orig = false;
 
-			list_for_each_entry(alt, &insn->alts, list) {
+			for (alt = insn->alts; alt; alt = alt->next) {
 				if (alt->skip_orig)
 					skip_orig = true;
 

tools/objtool/include/objtool/check.h

@@ -66,7 +66,7 @@ struct instruction {
 	struct instruction *first_jump_src;
 	struct reloc *jump_table;
 	struct reloc *reloc;
-	struct list_head alts;
+	struct alternative *alts;
 	struct symbol *sym;
 	struct stack_op *stack_ops;
 	struct cfi_state *cfi;