Implement variable length arrays

Variably modified types are required for C23.

Since QBE doesn't currently support saving and restoring the stack
pointer, a current limitation is that we can't reclaim stack space
from VLAs that go out of scope. This is potentially problematic for
VLAs appearing in a loop, but this case is uncommon enough that it
is silently ignored for now.

Implements: https://todo.sr.ht/~mcf/cproc/1
References: https://todo.sr.ht/~mcf/cproc/88
Co-authored-by: Michael Forney <mforney@mforney.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/test/builtin-vaarg-vm.c b/test/builtin-vaarg-vm.c
new file mode 100644
index 0000000..aee7216
--- /dev/null
+++ b/test/builtin-vaarg-vm.c
@@ -0,0 +1,16 @@
+int f(int i, ...) {
+	int r, c = 0;
+	__builtin_va_list ap;
+
+	__builtin_va_start(ap, i);
+	r = **__builtin_va_arg(ap, int (*)[++i]);
+	__builtin_va_end(ap);
+	return r + i;
+}
+
+int main(void) {
+	int a[3];
+
+	a[0] = 123;
+	return f(3, &a) != 127;
+}