https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91937
Bug ID: 91937
Summary: _Generic Feature Expansion
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: srinath.parvathaneni at arm dot com
Target Milestone: ---
Created attachment 46983
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46983&action=edit
Preprocessor output
On compiling the following test.c with gcc and checking the preprocessor file.
In C, for a macro using "_Generic" feature, "_Generic" expands completely but
not just for the matching case. This expansion is increasing the pre-processor
file size enormously in case of nested macros and slowing down the compiler.
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/5/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
5.4.0-6ubuntu1~16.04.11' --with-bugurl=file:///usr/share/doc/gcc-5/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-5 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-5-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-5-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-5-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.11)
$ cat test.c
enum {
type_int_t = 1,
type_char_t,
type_float_t,
type_int_p_t,
type_float_p_t,
type_char_p_t,
type_int_16,
type_int_32
};
float bb;
char cc;
#define _TYPE(x) _Generic(x, \
int *: type_int_p_t, \
float *: type_float_p_t, \
char *: type_char_p_t, \
default: _Generic(x, \
short: type_int_16, \
long: type_int_32,\
int: type_int_t, \
char: type_char_t, \
float: type_float_t))
extern void *__undef;
#define cast(param, type) \
_Generic(param, type: param, default: *(type *)__undef)
#define reg(p0) _Generic( \
(int (*)[_TYPE(p0)])0, \
int (*)[type_int_t]: reg_s8(cast(p0, int)), \
int (*)[type_char_t]: reg_f8(cast(p0, char)), \
int (*)[type_float_t]: reg_c8(cast(p0, float)), \
int (*)[type_int_p_t]: reg_sp(cast(p0, int *)), \
int (*)[type_float_p_t]: reg_fp(cast(p0, float *)), \
int (*)[type_char_p_t]: reg_cp(cast(p0, char *)))
float
reg_s8(int a)
{
return bb;
}
void reg_f8(float a){}
void reg_c8(char a){}
void reg_sp(int *a){}
void reg_fp(float *a){}
void reg_cp(char *a){}
int main()
{
int a;
float b;
char c;
reg(a);
reg(reg(a)); // more levels of nesting, more complex preprocessor output.
return 0;
}
$ gcc test.c --save-temps
