GCC Quad大小32和64位

Question

I'm using GCC compiler on Linux.
I want to use 64 bit compiler and build some of my files with -m32 option (32 bit compatibility). I found strange (for me) behavior of compiler.
My code is simple:

int main () {
   int test =(((0) & 0x00000000) << (32));
   return 0;
}

And for this code I received warning for 32 , 64 and 64 with -m32 compilers:

warning: left shift count >= width of type [enabled by default]

But when I'm trying to compile it from assebmly code :

.quad (((0) & (0x00000000)) << (32))

I received warning only for 32 bit compiler:

Warning: shift count out of range (32 is not between 0 and 31)

Why warnings are differ for same code compiled from c and asm file for different compilers (32 and 64 bit)?

EDIT:

Where can I find .quad definition, i mean in code?

Answer 1

In 64 bit mode, gcc uses the LP64 convention. This means that long and pointers are 64 bits wide, but int is still only 32 bits wide. In the expression

  int test =(((0) & 0x00000000) << (32));

the constants are all assumed to be ints , because int is big enough to hold them all, and int s are always only 32 bits. The result of a shift >= width of the type is UB in C because the behaviour is generally dependent on the underlying architecture's shift instruction.

Suppress the warning by forcing one of the numbers to be 64 bit.

long test =(((0) & 0x00000000L) << (32)); // Still gives a warning with -m32

or

long long test =(((0) & 0x00000000LL) << (32)); // No warning, long long is always 64 bit

Answer 2

In C, the type of the left operand of the shift is int , which is 32-bit on both platforms in question, so the shift is always undefined behaviour in terms of the C language.

I imagine that in your assembler the operand is either 32 or 64 bit, respectively.