From ba66d8f69edd895682f2661524a1d07612cb9ba8 Mon Sep 17 00:00:00 2001 From: cinap_lenrek Date: Fri, 30 Dec 2022 18:36:13 +0000 Subject: libc: fix 32-bit arch vlong->double conversion (thans cosa) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- sys/src/libc/386/vlrt.c | 10 +++++----- sys/src/libc/68000/vlrt.c | 10 +++++----- sys/src/libc/68020/vlrt.c | 10 +++++----- sys/src/libc/arm/vlrt.c | 10 +++++----- sys/src/libc/mips/vlrt.c | 10 +++++----- sys/src/libc/power/vlrt.c | 10 +++++----- sys/src/libc/sparc/vlrt.c | 10 +++++----- sys/src/libc/spim/vlrt.c | 10 +++++----- 8 files changed, 40 insertions(+), 40 deletions(-) (limited to 'sys/src/libc') diff --git a/sys/src/libc/386/vlrt.c b/sys/src/libc/386/vlrt.c index 132a04f99..b27705baf 100644 --- a/sys/src/libc/386/vlrt.c +++ b/sys/src/libc/386/vlrt.c @@ -95,12 +95,12 @@ _f2v(Vlong *y, float f) 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; diff --git a/sys/src/libc/68000/vlrt.c b/sys/src/libc/68000/vlrt.c index 646c1796f..12975b656 100644 --- a/sys/src/libc/68000/vlrt.c +++ b/sys/src/libc/68000/vlrt.c @@ -120,12 +120,12 @@ _f2v(Vlong *y, float f) 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; diff --git a/sys/src/libc/68020/vlrt.c b/sys/src/libc/68020/vlrt.c index 629449c24..bbd4179e7 100644 --- a/sys/src/libc/68020/vlrt.c +++ b/sys/src/libc/68020/vlrt.c @@ -120,12 +120,12 @@ _f2v(Vlong *y, float f) 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; diff --git a/sys/src/libc/arm/vlrt.c b/sys/src/libc/arm/vlrt.c index 606d5dfd1..aa0a8874c 100644 --- a/sys/src/libc/arm/vlrt.c +++ b/sys/src/libc/arm/vlrt.c @@ -80,12 +80,12 @@ _f2v(Vlong *y, float f) 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; diff --git a/sys/src/libc/mips/vlrt.c b/sys/src/libc/mips/vlrt.c index 81eaec7c7..a8725d89f 100644 --- a/sys/src/libc/mips/vlrt.c +++ b/sys/src/libc/mips/vlrt.c @@ -123,12 +123,12 @@ _f2v(Vlong *y, float f) 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; diff --git a/sys/src/libc/power/vlrt.c b/sys/src/libc/power/vlrt.c index e9fd3e8bf..9283c8e9c 100644 --- a/sys/src/libc/power/vlrt.c +++ b/sys/src/libc/power/vlrt.c @@ -82,12 +82,12 @@ _f2v(Vlong *y, float f) 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; diff --git a/sys/src/libc/sparc/vlrt.c b/sys/src/libc/sparc/vlrt.c index c1183beb9..2cc9ca500 100644 --- a/sys/src/libc/sparc/vlrt.c +++ b/sys/src/libc/sparc/vlrt.c @@ -124,12 +124,12 @@ _f2v(Vlong *y, float f) 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; diff --git a/sys/src/libc/spim/vlrt.c b/sys/src/libc/spim/vlrt.c index cfa7e4ca7..1f5c629fd 100644 --- a/sys/src/libc/spim/vlrt.c +++ b/sys/src/libc/spim/vlrt.c @@ -123,12 +123,12 @@ _f2v(Vlong *y, float f) 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; -- cgit v1.2.3