ref: ba66d8f69edd895682f2661524a1d07612cb9ba8
parent: b81609d071833c6d950af7a4efac53af86c7c276
author: cinap_lenrek <cinap_lenrek@felloff.net>
date: Fri Dec 30 13:36:13 EST 2022
libc: fix 32-bit arch vlong->double conversion (thans cosa) Cosa wrote: While trying to run kvik's lu9 on ARM, I found that when converting LLONG_MIN to a double on 32bit systems, the result is wrong (positive instead of negative). Given the following test program: void main() { vlong min = LLONG_MIN; double dmin = min; print("minint %lld\n", min); print("minint as double %f\n", dmin); if (dmin > 0.0) { exits("int min as double turned positive"); } exits(0); } The output on x86_64 will be: minint -9223372036854775808 minint as double -9223372036854776400.000000 But on arm or 386 (and I expect also spim, 68000, mips, 68020, sparc, power, since they all use the same _v2d): minint -9223372036854775808 minint as double 9223372036854776400.000000 And the value turned positive in the conversion. The function used for the cast to double is (in /sys/src/libc/arm/vlrt.c): double _v2d(Vlong x) { if(x.hi & SIGN(32)) { if(x.lo) { x.lo = -x.lo; x.hi = ~x.hi; } else x.hi = -x.hi; return -((long)x.hi*4294967296. + x.lo); } return (long)x.hi*4294967296. + x.lo; } If I understand correctly, the issue is that where it tries to flip the sign for x.hi (x.hi = -x.hi), 0x80000000 has no positive, thus stays the same (it stays negative). Then when we get to the negative return, we get a positive out. What came to my mind then, is that in the case that there is no x.lo, we can keep the x.hi sign and cast directly, thus: double _v2d(Vlong x) { if(!x.lo) { return (long)x.hi*4294967296.; } if(x.hi & SIGN(32)) { x.lo = -x.lo; x.hi = ~x.hi; return -((long)x.hi*4294967296. + x.lo); } return (long)x.hi*4294967296. + x.lo; } This looks correct to me, but I don't trust myself to not make mistakes in such critical code, so I would like some feedback on the change. Happy new year in advance, cosa
--- a/sys/src/libc/386/vlrt.c
+++ b/sys/src/libc/386/vlrt.c
@@ -95,12 +95,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
--- a/sys/src/libc/68000/vlrt.c
+++ b/sys/src/libc/68000/vlrt.c
@@ -120,12 +120,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
--- a/sys/src/libc/68020/vlrt.c
+++ b/sys/src/libc/68020/vlrt.c
@@ -120,12 +120,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
--- a/sys/src/libc/arm/vlrt.c
+++ b/sys/src/libc/arm/vlrt.c
@@ -80,12 +80,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
--- a/sys/src/libc/mips/vlrt.c
+++ b/sys/src/libc/mips/vlrt.c
@@ -123,12 +123,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
--- a/sys/src/libc/power/vlrt.c
+++ b/sys/src/libc/power/vlrt.c
@@ -82,12 +82,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
--- a/sys/src/libc/sparc/vlrt.c
+++ b/sys/src/libc/sparc/vlrt.c
@@ -124,12 +124,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
--- a/sys/src/libc/spim/vlrt.c
+++ b/sys/src/libc/spim/vlrt.c
@@ -123,12 +123,12 @@
double
_v2d(Vlong x)
{
+ if(!x.lo) {
+ return (long)x.hi*4294967296.;
+ }
if(x.hi & SIGN(32)) {
- if(x.lo) {
- x.lo = -x.lo;
- x.hi = ~x.hi;
- } else
- x.hi = -x.hi;
+ x.lo = -x.lo;
+ x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;