[musl] Wrong rounding in printf when precision is not set to max

[John M <johnm@braincalibration.de>]

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;
 	}
--