[PATCH] target/hppa: correct size bit parity for fmpyadd

[Gabriel Brookman <brookmangabriel@gmail.com>]

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. 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
---
Hi all,

This patch fixes the decoding of the fmpyadd instruction on the hppa
target, which uses an inverted bit convention to select between
32-bit and 64-bit floating-point registers. The issue was reported by
Andreas Hüttel after observing incorrect behavior when running real
binaries under that target. He kindly submitted a minimal reproducer
which I was able to use to debug the issue. I used this reproducer
to verify correct operation after my fix.

Thanks,
Gabriel

Reported-by: Andreas Hüttel
---
 target/hppa/insns.decode | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/target/hppa/insns.decode b/target/hppa/insns.decode
index 4eaac750ea..13c6a55bf2 100644
--- a/target/hppa/insns.decode
+++ b/target/hppa/insns.decode
@@ -365,10 +365,10 @@ fstd            011100 ..... ..... .. ............1.    @ldstim11
 &mpyadd         rm1 rm2 ta ra tm
 @mpyadd         ...... rm1:5 rm2:5 ta:5 ra:5 . tm:5     &mpyadd
 
-fmpyadd_f       000110 ..... ..... ..... ..... 0 .....  @mpyadd
-fmpyadd_d       000110 ..... ..... ..... ..... 1 .....  @mpyadd
-fmpysub_f       100110 ..... ..... ..... ..... 0 .....  @mpyadd
-fmpysub_d       100110 ..... ..... ..... ..... 1 .....  @mpyadd
+fmpyadd_f       000110 ..... ..... ..... ..... 1 .....  @mpyadd
+fmpyadd_d       000110 ..... ..... ..... ..... 0 .....  @mpyadd
+fmpysub_f       100110 ..... ..... ..... ..... 1 .....  @mpyadd
+fmpysub_d       100110 ..... ..... ..... ..... 0 .....  @mpyadd
 
 ####
 # Conditional Branches

---
base-commit: 94474a7733a57365d5a27efc28c05462e90e8944
change-id: 20251009-hppa-correct-fmpyadd-size-bit-decoding-059501a0ae49

Best regards,
-- 
Gabriel Brookman <brookmangabriel@gmail.com>