From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59498) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VuTmg-0000BZ-4R for qemu-devel@nongnu.org; Sat, 21 Dec 2013 16:08:34 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1VuTmd-0001j3-Qa for qemu-devel@nongnu.org; Sat, 21 Dec 2013 16:08:30 -0500 Received: from mnementh.archaic.org.uk ([2001:8b0:1d0::1]:43779) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1VuTmd-0001gR-JR for qemu-devel@nongnu.org; Sat, 21 Dec 2013 16:08:27 -0500 From: Peter Maydell Date: Sat, 21 Dec 2013 21:08:19 +0000 Message-Id: <1387660099-22390-1-git-send-email-peter.maydell@linaro.org> Subject: [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Richard Henderson , patches@linaro.org If the input to float*_scalbn() is denormal then it represents a number 0.[mantissabits] * 2^(1-exponentbias) (and the actual exponent field is all zeroes). This means that when we convert it to our unpacked encoding the unpacked exponent must be one greater than for a normal number, which represents 1.[mantissabits] * 2^(e-exponentbias) for an exponent field e. This meant we were giving answers too small by a factor of 2 for all denormal inputs. Note that the float-to-int routines also have this behaviour of not adjusting the exponent for denormals; however there it is harmless because denormals will all convert to integer zero anyway. Signed-off-by: Peter Maydell --- Changes v1->v2: propagated fix to 80 bit and 128 bit float functions. These function names remind me of Ken Thompson's reply to a question about what he'd do differently if he were redesigning UNIX: "I'd spell creat with an e." fpu/softfloat.c | 29 +++++++++++++++++++++-------- 1 file changed, 21 insertions(+), 8 deletions(-) diff --git a/fpu/softfloat.c b/fpu/softfloat.c index dbda61b..7f5e8c7 100644 --- a/fpu/softfloat.c +++ b/fpu/softfloat.c @@ -6795,10 +6795,13 @@ float32 float32_scalbn( float32 a, int n STATUS_PARAM ) } return a; } - if ( aExp != 0 ) + if (aExp != 0) { aSig |= 0x00800000; - else if ( aSig == 0 ) + } else if (aSig == 0) { return a; + } else { + aExp++; + } if (n > 0x200) { n = 0x200; @@ -6828,10 +6831,13 @@ float64 float64_scalbn( float64 a, int n STATUS_PARAM ) } return a; } - if ( aExp != 0 ) + if (aExp != 0) { aSig |= LIT64( 0x0010000000000000 ); - else if ( aSig == 0 ) + } else if (aSig == 0) { return a; + } else { + aExp++; + } if (n > 0x1000) { n = 0x1000; @@ -6861,8 +6867,12 @@ floatx80 floatx80_scalbn( floatx80 a, int n STATUS_PARAM ) return a; } - if (aExp == 0 && aSig == 0) - return a; + if (aExp == 0) { + if (aSig == 0) { + return a; + } + aExp++; + } if (n > 0x10000) { n = 0x10000; @@ -6891,10 +6901,13 @@ float128 float128_scalbn( float128 a, int n STATUS_PARAM ) } return a; } - if ( aExp != 0 ) + if (aExp != 0) { aSig0 |= LIT64( 0x0001000000000000 ); - else if ( aSig0 == 0 && aSig1 == 0 ) + } else if (aSig0 == 0 && aSig1 == 0) { return a; + } else { + aExp++; + } if (n > 0x10000) { n = 0x10000; -- 1.8.5