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Re: nano printf + powerpc gcc
Okay, attached is a quick simulation - using a similar code that uses
va_arg in the caller then either passes a copy of the va_list to a
callee, or passes it by pointer.
When compiled with -DUSE_VA_POINTERS, either for x86_64 or for i386, it
prints correct values from both f1/f2 for all ten iterations:
f2#0: 5
f1#0: 0.500000
f2#1: 5
f1#1: 0.500000
f2#2: 5
f1#2: 0.500000
f2#3: 5
f1#3: 0.500000
f2#4: 5
f1#4: 0.500000
f2#5: 5
f1#5: 0.500000
f2#6: 5
f1#6: 0.500000
f2#7: 5
f1#7: 0.500000
f2#8: 5
f1#8: 0.500000
f2#9: 5
f1#9: 0.500000
However, when passing a va_copy'ed va_list to the callee, it starts to
print junk. For x86_64, this happens when it runs out of registers to
pass the values - look at the last 3 lines:
f2#0: 5
f1#0: 0.500000
f2#1: 5
f1#1: 0.500000
f2#2: 5
f1#2: 0.500000
f2#3: 5
f1#3: 0.500000
f2#4: 5
f1#4: 0.500000
f2#5: 5
f1#5: 0.500000
f2#6: 5
f1#6: 0.500000
f2#7: 5
f1#7: 0.500000
f2#8: 5
f1#8: 0.000000
f2#9: 0
f1#9: 0.000000
It becomes even more pronounced on i386 where both integers and floating
point arguments are placed into the stack:
f2#0: 5
f1#0: 0.000000
f2#1: 0
f1#1: 0.000000
f2#2: 1071644672
f1#2: 0.000000
f2#3: 5
f1#3: 0.000000
f2#4: 0
f1#4: 0.000000
f2#5: 1071644672
f1#5: 0.000000
f2#6: 5
f1#6: 0.000000
f2#7: 0
f1#7: 0.000000
f2#8: 1071644672
f1#8: 0.000000
f2#9: 5
f1#9: 0.000000
So no, va_copy-based approach does not work where the callee needs to
*modify* the current state of the va_list in the caller.
Regards,
Alexey.
On 01/29/2018 12:45 PM, Jon Beniston wrote:
Hi Alexey,
That works okay for me.
Cheers,
Jon
-----Original Message-----
From: newlib-owner@sourceware.org [mailto:newlib-owner@sourceware.org] On Behalf Of Alexey Neyman
Sent: 29 January 2018 20:10
To: Eric Blake; Jon Beniston; 'Alexander Fedotov'; 'Andre Vieira (lists)'
Cc: newlib@sourceware.org
Subject: Re: nano printf + powerpc gcc
Wasn't the purpose of passing down `va_list *` to allow the callee to modify the current state of the varargs parser in the caller? As far as I understand, with the va_copy the callee (_printf_float) will get the arguments from the beginning of the varargs, disregarding the arguments already fetched by the caller (_VFPRINTF_R).
I'd suggest to verify the corner cases where floating point arguments are intermixed with integer arguments and both exceed the number of the registers available for passing them - causing both to spill into the stack. On PPC, that would be more than 8 integer and 8 floating point arguments, as far as I remember the ABI. Something like:
printf("%u %f %u %f %u %f %u %f %u %f %u %f %u %f %u %f %u %f %u %f\n",
5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5,
5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5);
Regards,
Alexey.
On 01/29/2018 07:48 AM, Eric Blake wrote:
On 01/29/2018 05:56 AM, Jon Beniston wrote:
Hi,
Is this patch what is recommended? Seems to fix it for me (but only very briefly tested on my target).
@@ -485,6 +475,7 @@ _VFPRINTF_R (struct _reent *data,
register char *cp; /* Handy char pointer (short term usage). */
const char *flag_chars;
struct _prt_data_t prt_data; /* All data for decoding format string. */
+ va_list ap_copy;
/* Output function pointer. */
int (*pfunc)(struct _reent *, FILE *, const char *, size_t len);
@@ -522,6 +513,8 @@ _VFPRINTF_R (struct _reent *data,
prt_data.blank = ' ';
prt_data.zero = '0';
+ va_copy (ap_copy, ap);
+
/* Scan the format for conversions (`%' character). */
for (;;)
{
@@ -577,7 +570,7 @@ _VFPRINTF_R (struct _reent *data,
* -- ANSI X3J11
* They don't exclude field widths read from args.
*/
- prt_data.width = GET_ARG (n, ap, int);
+ prt_data.width = GET_ARG (n, ap_copy, int);
...
else
- {
- n = _printf_float (data, &prt_data, fp, pfunc, va_ptr(ap));
- }
+ n = _printf_float (data, &prt_data, fp, pfunc,
+ &ap_copy);
Maybe a comment why the copy is needed is still in order, but yes,
this matches what I was thinking.
#include <stdio.h>
#include <stdarg.h>
static void f1(int i,
#if defined(USE_VA_POINTERS)
va_list *p_ap
#else
va_list ap
#endif
)
{
double x;
#if defined(USE_VA_POINTERS)
x = va_arg(*p_ap, double);
#else
x = va_arg(ap, double);
#endif
printf("%s#%u: %f\n", __func__, i, x);
}
static void f2(va_list ap)
{
int i, x;
#if !defined(USE_VA_POINTERS)
va_list ap2;
va_copy(ap2, ap);
#endif
for (i = 0; i < 10; i++) {
x = va_arg(ap, int);
printf("%s#%u: %u\n", __func__, i, x);
#if defined(USE_VA_POINTERS)
#if defined(__x86_64__)
f1(i, (va_list *)ap);
#elif defined(__i386__)
f1(i, &ap);
#else
#error How to pass va_list pointers on this architecture?
#endif
#else
f1(i, ap2);
#endif
}
}
void fx(int dummy, ...)
{
va_list ap;
va_start(ap, dummy);
f2(ap);
va_end(ap);
}
int main(void)
{
fx(0, 5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5,
5, 0.5, 5, 0.5, 5, 0.5, 5, 0.5);
return 0;
}