[PATCH 69/73] builtin: Do not evaluate __builtin_clz(0) and __builtin_ctz(0)

[Bart Van Assche <bvanassche@acm.org>]

Do not evaluate __builtin_clz() and __builtin_ctz() at compile time when
the argument is zero. Return SIDE_EFFECTS instead so that the expression
is not constant-folded.

Avoid undefined behavior in the host compiler. Prevent crashes when
sparse is built with -fsanitize=undefined and run with
UBSAN_OPTIONS=halt_on_error=1, as passing zero to the host compiler
builtins invokes undefined behavior.

Add a regression test in validation/builtin-clz-ctz-zero.c to verify that
passing zero to __builtin_clz() and __builtin_ctz() does not cause sparse
to crash.

Fixes: 3df9233e87de ("expand more builtins like __builtin_ffs()")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 builtin.c                         | 16 +++++++++-------
 validation/builtin-clz-ctz-zero.c | 11 +++++++++++
 2 files changed, 20 insertions(+), 7 deletions(-)
 create mode 100644 validation/builtin-clz-ctz-zero.c

diff --git a/builtin.c b/builtin.c
index 34af6d82b846..93377718f1fc 100644
--- a/builtin.c
+++ b/builtin.c
@@ -246,7 +246,7 @@ static struct symbol_op bswap_op = {
 };
 
 
-#define EXPAND_FINDBIT(name)					\
+#define EXPAND_FINDBIT(name, zero_is_ub)			\
 static int expand_##name(struct expression *expr, int cost)	\
 {								\
 	struct expression *arg;					\
@@ -257,6 +257,8 @@ static int expand_##name(struct expression *expr, int cost)	\
 								\
 	arg = first_expression(expr->args);			\
 	val = get_expression_value_silent(arg);			\
+	if (zero_is_ub && !val)					\
+		return SIDE_EFFECTS;				\
 	switch (arg->ctype->bit_size) {				\
 	case sizeof(int) * 8:					\
 		val = __builtin_##name(val); break;		\
@@ -277,12 +279,12 @@ static struct symbol_op name##_op = {				\
 	.expand = expand_##name,				\
 }
 
-EXPAND_FINDBIT(clz);
-EXPAND_FINDBIT(ctz);
-EXPAND_FINDBIT(clrsb);
-EXPAND_FINDBIT(ffs);
-EXPAND_FINDBIT(parity);
-EXPAND_FINDBIT(popcount);
+EXPAND_FINDBIT(clz, 1);
+EXPAND_FINDBIT(ctz, 1);
+EXPAND_FINDBIT(clrsb, 0);
+EXPAND_FINDBIT(ffs, 0);
+EXPAND_FINDBIT(parity, 0);
+EXPAND_FINDBIT(popcount, 0);
 
 static int evaluate_fp_unop(struct expression *expr)
 {
diff --git a/validation/builtin-clz-ctz-zero.c b/validation/builtin-clz-ctz-zero.c
new file mode 100644
index 000000000000..eb49f0c175d9
--- /dev/null
+++ b/validation/builtin-clz-ctz-zero.c
@@ -0,0 +1,11 @@
+static int ctz_zero = __builtin_ctz(0);
+static int clz_zero = __builtin_clz(0);
+static int ctzll_zero = __builtin_ctzll(0);
+static int clzll_zero = __builtin_clzll(0);
+
+/*
+ * check-name: __builtin_clz and __builtin_ctz with zero
+ * check-command: sparse -Wno-decl $file
+ * check-error-start
+ * check-error-end
+ */