s390/bpf: Change seen_reg to a mask

Using a mask instead of an array saves a small amount of memory and
allows marking multiple registers as seen with a simple "or". Another
positive side-effect is that it speeds up verification with jitterbug.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240703005047.40915-2-iii@linux.ibm.com

arch/s390/net/bpf_jit_comp.c

@@ -35,7 +35,7 @@
 
 struct bpf_jit {
 	u32 seen;		/* Flags to remember seen eBPF instructions */
-	u32 seen_reg[16];	/* Array to remember which registers are used */
+	u16 seen_regs;		/* Mask to remember which registers are used */
 	u32 *addrs;		/* Array with relative instruction addresses */
 	u8 *prg_buf;		/* Start of program */
 	int size;		/* Size of program and literal pool */
@@ -120,8 +120,8 @@ static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
 {
 	u32 r1 = reg2hex[b1];
 
-	if (r1 >= 6 && r1 <= 15 && !jit->seen_reg[r1])
-		jit->seen_reg[r1] = 1;
+	if (r1 >= 6 && r1 <= 15)
+		jit->seen_regs |= (1 << r1);
 }
 
 #define REG_SET_SEEN(b1)					\
@@ -129,8 +129,6 @@ static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
 	reg_set_seen(jit, b1);					\
 })
 
-#define REG_SEEN(b1) jit->seen_reg[reg2hex[(b1)]]
-
 /*
  * EMIT macros for code generation
  */
@@ -438,12 +436,12 @@ static void restore_regs(struct bpf_jit *jit, u32 rs, u32 re, u32 stack_depth)
 /*
  * Return first seen register (from start)
  */
-static int get_start(struct bpf_jit *jit, int start)
+static int get_start(u16 seen_regs, int start)
 {
 	int i;
 
 	for (i = start; i <= 15; i++) {
-		if (jit->seen_reg[i])
+		if (seen_regs & (1 << i))
 			return i;
 	}
 	return 0;
@@ -452,15 +450,15 @@ static int get_start(struct bpf_jit *jit, int start)
 /*
  * Return last seen register (from start) (gap >= 2)
  */
-static int get_end(struct bpf_jit *jit, int start)
+static int get_end(u16 seen_regs, int start)
 {
 	int i;
 
 	for (i = start; i < 15; i++) {
-		if (!jit->seen_reg[i] && !jit->seen_reg[i + 1])
+		if (!(seen_regs & (3 << i)))
 			return i - 1;
 	}
-	return jit->seen_reg[15] ? 15 : 14;
+	return (seen_regs & (1 << 15)) ? 15 : 14;
 }
 
 #define REGS_SAVE	1
@@ -469,8 +467,10 @@ static int get_end(struct bpf_jit *jit, int start)
  * Save and restore clobbered registers (6-15) on stack.
  * We save/restore registers in chunks with gap >= 2 registers.
  */
-static void save_restore_regs(struct bpf_jit *jit, int op, u32 stack_depth)
+static void save_restore_regs(struct bpf_jit *jit, int op, u32 stack_depth,
+			      u16 extra_regs)
 {
+	u16 seen_regs = jit->seen_regs | extra_regs;
 	const int last = 15, save_restore_size = 6;
 	int re = 6, rs;
 
@@ -484,10 +484,10 @@ static void save_restore_regs(struct bpf_jit *jit, int op, u32 stack_depth)
 	}
 
 	do {
-		rs = get_start(jit, re);
+		rs = get_start(seen_regs, re);
 		if (!rs)
 			break;
-		re = get_end(jit, rs + 1);
+		re = get_end(seen_regs, rs + 1);
 		if (op == REGS_SAVE)
 			save_regs(jit, rs, re);
 		else
@@ -573,7 +573,7 @@ static void bpf_jit_prologue(struct bpf_jit *jit, struct bpf_prog *fp,
 	/* Tail calls have to skip above initialization */
 	jit->tail_call_start = jit->prg;
 	/* Save registers */
-	save_restore_regs(jit, REGS_SAVE, stack_depth);
+	save_restore_regs(jit, REGS_SAVE, stack_depth, 0);
 	/* Setup literal pool */
 	if (is_first_pass(jit) || (jit->seen & SEEN_LITERAL)) {
 		if (!is_first_pass(jit) &&
@@ -649,7 +649,7 @@ static void bpf_jit_epilogue(struct bpf_jit *jit, u32 stack_depth)
 	/* Load exit code: lgr %r2,%b0 */
 	EMIT4(0xb9040000, REG_2, BPF_REG_0);
 	/* Restore registers */
-	save_restore_regs(jit, REGS_RESTORE, stack_depth);
+	save_restore_regs(jit, REGS_RESTORE, stack_depth, 0);
 	if (nospec_uses_trampoline()) {
 		jit->r14_thunk_ip = jit->prg;
 		/* Generate __s390_indirect_jump_r14 thunk */
@@ -1847,7 +1847,7 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		/*
 		 * Restore registers before calling function
 		 */
-		save_restore_regs(jit, REGS_RESTORE, stack_depth);
+		save_restore_regs(jit, REGS_RESTORE, stack_depth, 0);
 
 		/*
 		 * goto *(prog->bpf_func + tail_call_start);