https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106767
Bug ID: 106767
Summary: Failure to detect recursive macro calls due to
_Pragma(pop_macro)
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: preprocessor
Assignee: unassigned at gcc dot gnu.org
Reporter: izbyshev at ispras dot ru
CC: amonakov at gcc dot gnu.org
Target Milestone: ---
GCC preprocessor appears to behave as if each _Pragma(pop_macro("m")) creates a
new definition of "m", so, for example, the following code results in infinite
recursion:
#define P(x) _Pragma(#x)
#define f() P(push_macro("f")) P(pop_macro("f")) f()
f()
Naturally, this makes the preprocessor Turing-complete, e.g. it can add
numbers:
#define P(x) _Pragma(#x)
#define PUSH(x) P(push_macro(#x))
#define POP(x) P(pop_macro(#x))
#define R(f) PUSH(f) POP(f)
#define C(x, y) x ## y
#define ADD0(...) __VA_ARGS__
#define ADD1(...) 1, R(ADD) ADD(__VA_ARGS__)
#define ADD(x, ...) C(ADD, x)(__VA_ARGS__)
// 1 + 2 (zero-terminated unary numbers)
ADD(1, 0, 1, 1, 0)
// Outputs 1, 1, 1, 0 with "gcc -E -P"
The earliest GCC with this behavior on https://godbolt.org is 4.4.7.
Clang detects recursion correctly.
