From 232b6ec5df874236166fb0167cd473601a631715 Mon Sep 17 00:00:00 2001 From: "Maciej W. Rozycki" Date: Fri, 3 Apr 2015 23:27:43 +0100 Subject: [PATCH] MIPS: math-emu: Make ABS.fmt and NEG.fmt arithmetic again The ABS.fmt and NEG.fmt instructions have been specified as arithmetic in the MIPS architecture, which in particular implies handling NaN data in the usual way with qNaN bit patterns propagated unchanged and sNaN bit patterns signalling the usual IEEE 754 Invalid Operation exception and quieted by default. A series of changes applied over time to our implementation: c5033d78 [MIPS] ieee754[sd]p_neg workaround cea2be44 MIPS: Fix abs.[sd] and neg.[sd] emulation for NaN operands has led to the current situation where the sign bit is updated according to the operation requested even for NaN inputs. This is according to these commits a workaround so that broken binaries produced by GCC disregarding the properties of these instructions have a chance to work. For sNaN inputs this remains within IEEE Std 754 as the standard leaves the choice of output qNaN bit patterns produced under the default Invalid Operation exception handling for individual sNaN input bit patterns to implementer's discretion, even though it still recommends as much NaN input information to be preserved in NaN outputs. For qNaN inputs however it violates the standard as it requires a qNaN input bit patterns to propagate unchanged to output. This is also unlike real MIPS FPU hardware behaves where sNaN and/or qNaN processing has been fully implemented with no Unimplemented Operation exception signalled. Such hardware propagates any input qNaN bit pattern unchanged. It also quiets any input sNaN bit pattern in an implementer-specific manner, for example the MIPS 74Kf processor returns the default qNaN pattern with the sign bit always clear and the Broadcom SB-1 and BMIPS5000 processors propagate the input sNaN bit pattern with the sign bit unchanged and the quiet bit first cleared in the trailing significand field and then the next lower bit set if clearing the quiet bit left the field with no other bit set. Especially the latter observation indicates the limited usefulness of the workaround as it will cover many hardware configurations, but not all of them, only making it harder to discover such broken binaries that need to be recompiled with GCC told to avoid the use of ABS.fmt and NEG.fmt instructions where non-arithmetic semantics is required by the algorithm used. Revert the damage done by the series of changes then, and take the opportunity to simplify implementation by calling `ieee754dp_sub' and `ieee754dp_add' as required and also the rounding mode set towards -Inf temporarily so that the sign of 0 is correctly handled. Signed-off-by: Maciej W. Rozycki Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/9710/ Signed-off-by: Ralf Baechle --- arch/mips/math-emu/dp_simple.c | 55 ++++++++++++---------------------- arch/mips/math-emu/sp_simple.c | 55 ++++++++++++---------------------- 2 files changed, 38 insertions(+), 72 deletions(-) diff --git a/arch/mips/math-emu/dp_simple.c b/arch/mips/math-emu/dp_simple.c index bccbe90efceb..926d56bf37f2 100644 --- a/arch/mips/math-emu/dp_simple.c +++ b/arch/mips/math-emu/dp_simple.c @@ -23,44 +23,27 @@ union ieee754dp ieee754dp_neg(union ieee754dp x) { - COMPXDP; - - EXPLODEXDP; - ieee754_clearcx(); - FLUSHXDP; - - /* - * Invert the sign ALWAYS to prevent an endless recursion on - * pow() in libc. - */ - /* quick fix up */ - DPSIGN(x) ^= 1; - - if (xc == IEEE754_CLASS_SNAN) { - union ieee754dp y = ieee754dp_indef(); - ieee754_setcx(IEEE754_INVALID_OPERATION); - DPSIGN(y) = DPSIGN(x); - return ieee754dp_nanxcpt(y); - } - - return x; + unsigned int oldrm; + union ieee754dp y; + + oldrm = ieee754_csr.rm; + ieee754_csr.rm = FPU_CSR_RD; + y = ieee754dp_sub(ieee754dp_zero(0), x); + ieee754_csr.rm = oldrm; + return y; } union ieee754dp ieee754dp_abs(union ieee754dp x) { - COMPXDP; - - EXPLODEXDP; - ieee754_clearcx(); - FLUSHXDP; - - /* Clear sign ALWAYS, irrespective of NaN */ - DPSIGN(x) = 0; - - if (xc == IEEE754_CLASS_SNAN) { - ieee754_setcx(IEEE754_INVALID_OPERATION); - return ieee754dp_nanxcpt(ieee754dp_indef()); - } - - return x; + unsigned int oldrm; + union ieee754dp y; + + oldrm = ieee754_csr.rm; + ieee754_csr.rm = FPU_CSR_RD; + if (DPSIGN(x)) + y = ieee754dp_sub(ieee754dp_zero(0), x); + else + y = ieee754dp_add(ieee754dp_zero(0), x); + ieee754_csr.rm = oldrm; + return y; } diff --git a/arch/mips/math-emu/sp_simple.c b/arch/mips/math-emu/sp_simple.c index f1ffaa9a17e0..c50e9451f2d2 100644 --- a/arch/mips/math-emu/sp_simple.c +++ b/arch/mips/math-emu/sp_simple.c @@ -23,44 +23,27 @@ union ieee754sp ieee754sp_neg(union ieee754sp x) { - COMPXSP; - - EXPLODEXSP; - ieee754_clearcx(); - FLUSHXSP; - - /* - * Invert the sign ALWAYS to prevent an endless recursion on - * pow() in libc. - */ - /* quick fix up */ - SPSIGN(x) ^= 1; - - if (xc == IEEE754_CLASS_SNAN) { - union ieee754sp y = ieee754sp_indef(); - ieee754_setcx(IEEE754_INVALID_OPERATION); - SPSIGN(y) = SPSIGN(x); - return ieee754sp_nanxcpt(y); - } - - return x; + unsigned int oldrm; + union ieee754sp y; + + oldrm = ieee754_csr.rm; + ieee754_csr.rm = FPU_CSR_RD; + y = ieee754sp_sub(ieee754sp_zero(0), x); + ieee754_csr.rm = oldrm; + return y; } union ieee754sp ieee754sp_abs(union ieee754sp x) { - COMPXSP; - - EXPLODEXSP; - ieee754_clearcx(); - FLUSHXSP; - - /* Clear sign ALWAYS, irrespective of NaN */ - SPSIGN(x) = 0; - - if (xc == IEEE754_CLASS_SNAN) { - ieee754_setcx(IEEE754_INVALID_OPERATION); - return ieee754sp_nanxcpt(ieee754sp_indef()); - } - - return x; + unsigned int oldrm; + union ieee754sp y; + + oldrm = ieee754_csr.rm; + ieee754_csr.rm = FPU_CSR_RD; + if (SPSIGN(x)) + y = ieee754sp_sub(ieee754sp_zero(0), x); + else + y = ieee754sp_add(ieee754sp_zero(0), x); + ieee754_csr.rm = oldrm; + return y; } -- 2.39.5