Re: Calling convention that gets frame pointer register clobbered

[Etienne Lorrain <etienne_lorrain at yahoo dot fr>]

 --- Falk Hueffner wrote:
> You've hit PR inline-asm/8788. As I mentioned there, IMHO we should
> error out on this, but nobody else has commented on it yet...

  But inline-asm/8788 is said to be invalid code.
 Anything can be invalid-ated, but the code:

-------
typedef unsigned farptr;

typedef struct {
    unsigned short	buffersize;
    unsigned short	infobit;
    unsigned		nbcylinder;
    unsigned		nbhead;
    unsigned		nbsectorpertrack;
    unsigned long long	nbtotalsector;
    unsigned short	bytepersector;
    /* V2.0+ : */
    farptr	configptr; /* for instance ebiosPhoenix realmode address */
    /* V3.0+ : */
    unsigned short signature0xBEDD;
    unsigned char  lendevicepath; /* 0x24 for v3.0 */
    unsigned char  reserved[3];
    unsigned char  bustype[4]; /* exactly "ISA" or "PCI" */
    unsigned char  Interface[8]; /* exactly "ATA", "ATAPI", "SCSI",
"USB"...*/
    union interface_path_u {
	struct {
	    unsigned short addr;
	    unsigned char pad[6];
	    } __attribute__ ((packed)) isa;
	struct {
	    unsigned char busnr;
	    unsigned char devicenr;
	    unsigned char functionnr;
	    unsigned char pad[5];
	    } __attribute__ ((packed)) pci;
	} bus;
    union device_path_u {
	struct {
	    unsigned char slave; /* 0 if master, 1 if slave */
	    unsigned char pad[7];
	    unsigned long long	reserved; // Not present in some docs...
	  } __attribute__ ((packed)) ata;
	struct {
	    unsigned char slave; /* 0 if master, 1 if slave */
	    unsigned char logical_unit_nr;
	    unsigned char pad[6];
	    unsigned long long	reserved; // Not present in some docs...
	  } __attribute__ ((packed)) atapi;
	struct {
	    unsigned char logical_unit_nr;
	    unsigned char pad[7];
	    unsigned long long	reserved; // Not present in some docs...
	  } __attribute__ ((packed)) scsi;
	struct {
	    unsigned char tobedefined;
	    unsigned char pad[7];
	    unsigned long long	reserved; // Not present in some docs...
	  } __attribute__ ((packed)) usb;
	struct {
	    unsigned long long FireWireGeneralUniqueID;
	    unsigned long long	reserved; // Not present in some docs...
	  } __attribute__ ((packed)) ieee1394;
	struct {
	    unsigned long long WorldWideNumber;
	    unsigned long long	reserved; // Not present in some docs...
	  } __attribute__ ((packed)) FibreChannel;
      } device;
    unsigned char zero;
    unsigned char bytechecksum; /* byte sum [0x1E..0x49] = 0 */
    } __attribute__ ((packed)) ebiosinfo_t;

unsigned char
_EBIOSDISK_getparam (unsigned char disk, ebiosinfo_t *ebiosinfo,
                     unsigned char *status)
  {
  unsigned char carry;

  asm (
"       int     $0x13           # _EBIOSDISK_getparam   \n"
"       mov     %%ah,%1                                 \n"
"       setc    %0                                      \n"
        : "=qm" (carry), "=qm" (*status), "=S" (*ebiosinfo)
        : "a"((unsigned short)0x4800), "d" (disk)
        );

  return carry;
  }
-------

  This code has a real use in a real (at least for me) application...

$ gcc -O2 t.c
t.c: In function `_EBIOSDISK_getparam':
t.c:72: error: impossible constraint in `asm'

  The change of error is because it is _EBIOSDISK_getparam() so it
 is an output - for _EBIOSDISK_setparam() or equivalent it would
 be an input.
  Using output "=m" (*ebiosinfo) and input "S" (ebiosinfo) is not
 completely equivalent because the output part may generate strange
 code on some compiler version - but that is the best approximation.
 Using "memory" after ": : :" does not have the intended effect if
 the structure is declared locally in the current stack frame (and
 if the _EBIOSDISK_getparam() function is inlined).
  The system saying "if the structure fit in a register, it is
 inside the register - else the register contains its base address"
 seems to be already used in GCC when a real C function returns a
 structure (C extension of GCC).

  Etienne.



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