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Date: Mon, 1 Jan 2024 10:11:09 +0100
From: John M <johnm@...incalibration.de>
To: musl@...ts.openwall.com
Subject: Wrong rounding in printf when precision is not set to max
I noticed printf rounding wrongly when FPU Control Word[PC] != 11.
I do not think my workaround is a fix, it just serves to show where the
reduced precision breaks the rounding.
Do you consider this a bug?
If yes, do you have an idea or comments on how a real fix would look
like?
Test case:
---
// x86_64-pc-linux-gnu-gcc float.c -o float-glibc
// x86_64-pc-linux-musl-gcc float.c -o float-musl
#include <stdio.h>
#define STREFLOP_FSTCW(cw) do { asm volatile ("fstcw %0" : "=m" (cw) : ); } while (0)
#define STREFLOP_FLDCW(cw) do { asm volatile ("fclex \n fldcw %0" : : "m" (cw)); } while (0)
void cw_set(unsigned short x87_mode) {
STREFLOP_FLDCW(x87_mode);
}
void cw_print() {
unsigned short x87_mode;
STREFLOP_FSTCW(x87_mode);
printf("FPU Control Word: 0x%04X\n", x87_mode);
}
int main() {
cw_print();
printf("%11.3f\n", 1200.12345);
printf("%11.3f\n", 3.14159274);
cw_set(0x007F);
cw_print();
// FIXME: musl rounds this up to 1200.124 when FPU Control Word[PC] != 11. glibc does round correctly.
printf("%11.3f\n", 1200.12345);
// FIXME: musl rounds this down to 3.141 when FPU Control Word[PC] != 11. glibc does round correctly.
printf("%11.3f\n", 3.14159274);
return 0;
}
--
My workaround:
---
diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c
index 497c5e19..9b2bdb8c 100644
--- a/src/stdio/vfprintf.c
+++ b/src/stdio/vfprintf.c
@@ -13,6 +13,26 @@
/* Some useful macros */
+#define STREFLOP_FSTCW(cw) do { __asm__ volatile ("fstcw %0" : "=m" (cw) : ); } while (0)
+#define STREFLOP_FLDCW(cw) do { __asm__ volatile ("fclex \n fldcw %0" : : "m" (cw)); } while (0)
+
+unsigned short x87_mode_old;
+void cw_push(unsigned short flags) {
+ STREFLOP_FSTCW(x87_mode_old);
+ unsigned short x87_mode = x87_mode_old | flags;
+ STREFLOP_FLDCW(x87_mode);
+}
+
+void cw_pop() {
+ STREFLOP_FLDCW(x87_mode_old);
+}
+
+void cw_print() {
+ unsigned short x87_mode;
+ STREFLOP_FSTCW(x87_mode);
+ printf("FPU Control Word: 0x%04X\n", x87_mode);
+}
+
#define MAX(a,b) ((a)>(b) ? (a) : (b))
#define MIN(a,b) ((a)<(b) ? (a) : (b))
@@ -318,6 +338,7 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
x = *d % i;
/* Are there any significant digits past j? */
if (x || d+1!=z) {
+ cw_push(0x0300);
long double round = 2/LDBL_EPSILON;
long double small;
if ((*d/i & 1) || (i==1000000000 && d>a && (d[-1]&1)))
@@ -337,6 +358,7 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
}
for (i=10, e=9*(r-a); *a>=i; i*=10, e++);
}
+ cw_pop();
}
if (z>d+1) z=d+1;
}
--
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