[RFC][PATCH] fix casting constant to _Bool

[Xi Wang <xi.wang@gmail.com>]

Casting to _Bool requires a zero test rather than truncation.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
---
A simple example is:

  _Bool x = 0x200;

x should have been true rather than false.

BTW, this patch also disables the warning "cast truncates bits from
constant value".  Does that sound good?

A more serious problem is something like:

  _Bool foo(int x) { return x; }

sparse emits:

  scast.1     %r2 <- (32) %arg1

which makes sparse-llvm generate:

  %R2 = trunc i32 %0 to i1

My experimental backend "splay" has to treat _Bool specifically to
generate:

  %0 = icmp ne i32 %x, 0

Should we leave the conversion job to backends, or should we just fix
the sparse IR (e.g., emitting setne rather than scast for casts to _Bool)?
---
 expand.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/expand.c b/expand.c
index 63a9075..ee818a4 100644
--- a/expand.c
+++ b/expand.c
@@ -92,6 +92,12 @@ void cast_value(struct expression *expr, struct symbol *newtype,
 	value = get_longlong(old);
 
 Int:
+	// _Bool requires a zero test rather than truncation.
+	if (is_bool_type(newtype)) {
+		expr->value = value ? 1 : 0;
+		return;
+	}
+
 	// Truncate it to the new size
 	signmask = 1ULL << (new_size-1);
 	mask = signmask | (signmask-1);
-- 
1.7.9.5