For the fmpyadd instruction on the hppa architecture, there is a bit
used to specify whether the instruction is operating on a 32 bit or
64 bit floating point register. For most instructions, such a bit is 0
when operating on the smaller register and 1 when operating on the
larger register. However, according to page 6-57 of the PA-RISC 1.1
Architecture and Instruction Set Reference Manual, this convention is
reversed for the fmpyadd instruction specifically, meaning the bit is
1 for operations on 32 bit registers and 0 for 64 bit registers. See
also page 6-18 (fig. 6-8) and 6-19 (table 6-16), where the f field
for FMPYADD and FMPYSUB is documented. Previously, QEMU decoded this
operation as operating on the other size of register, leading to bugs
when translating the fmpyadd instruction. This patch fixes that issue.
Reported-by: Andreas Hüttel <andreas.huettel@ur.de>
Signed-off-by: Gabriel Brookman <brookmangabriel@gmail.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3096
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Helge Deller <deller@gmx.de>
Message-ID: <20251009-hppa-correct-fmpyadd-size-bit-decoding-v1-1-f63bb6c3290c@gmail.com>
[PMD: Add documentation refs mentioned by Andreas K. Huettel]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
cea82f8cdd