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GCC generates the wrong instructions when I write a function which returns a reference to a local variable?

+5 votes
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GCC generates the wrong instructions when I write a function which returns a reference to a local variable?
posted May 1, 2016 by anonymous

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Will you like to share more detail that what is the function, what is expected and what gcc is generating as instructions.

1 Answer

+1 vote

The compiler has generated the right instructions. What you are doing is not defined by the C standard, so the compiler is free to do whatever it pleases. In this case it seems that GCC pleases to return a null pointer, probably so that your program will fail as fast as possible.

answer May 1, 2016 by Rajan Paswan
Similar Questions
+5 votes

Even I have similar problem:

int (*pfun)(void);
int *pInt = 0;
void fun1()
{
    int i = 5; /* Local to fun1*/

    printf("Outer function");
    pInt = &i; /* As I know address of local variable is valid till function execution */

    int fun2()
    {
      printf("innerfunction");
      printf("%d", *pInt);
    }
    /* fun2 address assign to pfun so that It can be called even after completion of fun1 */
    pfun = fun2;
}

int main()
{
    fun1();
    pfun();
    return 0;
}

Can someone please explain ? I am getting *pInt value 5.

0 votes

I was working in an Embedded processor with GCC-4.6.4 version. I need to add load/store reverse instructions to the MD file. My instructions will look as below:

 LWX Rd,Ra,Rb
 operation: Addr := Ra + Rb
 Rd := *Addr (loading data with the opposite endianness)
 SWX Rd,Ra,Rb
 operation: Addr := Ra + Rb
 *Addr := Rd (storing data with the opposite endianness)

To add the above instructions in to md file I tried below pattern in md file

 (define_insn "movsi_rev"
 [(set (match_operand:SI 0 "nonimmediate_operand" "=d,m")
 (bswap: SI (match_operand:SI 1 "move_src_operand" "m,d")))]
 ""
 "@
 lwxt%0,%1,%0
 swxt%0,%1,%0"
 [(set_attr "type" "load,store")
 (set_attr "mode" "SI")
 (set_attr "length" "4,4")])

I wrote a small testcase which is generating swx instruction but the operands are more due to which it is failing in assembler phase

 ex: instead of swx r0,r2,r0 it is generating swx r0,r2,0,r0

can anyone please help me in removing the extra operand in the above instruction.

+1 vote

in the following code func.c :

 int Myfunc1(int i, int z)
 {
 return i;
 }

 int main()
 {
 int ans;

 /* casting the function into an 'int (int)' function */
 ans = ((int(*)(int))(Myfunc1))(5);

 printf("ans: %dnn", ans);

 return 0;
 }

I tried to cast an int(int,int) function into an int(int) function an got the gcc warning and note:

 func.c:13:32: warning: function called through a non-compatible type [enabled by default]
 func.c:13:32: note: if this code is reached, the program will abort

and when trying to run I get:

 Illegal instruction (core dumped)

But if i compile this file with a .cpp ending with the gcc compiler it works OK.

+1 vote

I wonder how one could get the compiler to generate the "movdqu" instruction, since the vector extensions always seem to assume that everything will be aligned to 16 byte.
I tried using a packed struct and this dint help much. Of course one can always resort to inline assembly but this should not be necessary

Compile with:

gcc -O2 -S -msse2 testvecs.c

Using built-in specs.

COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i486-linux-gnu/4.7/lto-wrapper
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-5' 
--with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs 
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr 
--program-suffix=-4.7 --enable-shared --enable-linker-build-id 
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext 
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7 
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu 
--enable-libstdcxx-debug --enable-libstdcxx-time=yes 
--enable-gnu-unique-object --enable-plugin --enable-objc-gc 
--enable-targets=all --with-arch-32=i586 --with-tune=generic 
--enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu 
--target=i486-linux-gnu
Thread model: posix
gcc version 4.7.2 (Debian 4.7.2-5)
0 votes

I'm studying about C compiler for increasing software quality. So I want to get all of compile error message list of gcc about C language. I was trying to find it. But I can't find it anywhere. How can I find it?

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