[Bug c/91937] New: _Generic Feature Expansion

[srinath.parvathaneni at arm dot com]

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