The FI bit in FPSCR is said to be a non-sticky bit on Power ISA.
One could think this means that, if an instruction is said to modify
the FPSCR register, the bit FI should be cleared. This is what QEMU
does today.
However, the following inconsistency was found when comparing results
from the hardware (tested on both a Power 9 processor and in
Power 10 Mambo):
(FI bit is set before the execution of the instruction)
Hardware: xscmpeqdp(0xff..ff, 0xff..ff) = FI: SET -> SET
QEMU: xscmpeqdp(0xff..ff, 0xff..ff) = FI: SET -> CLEARED
This is happening to multiple instructions in the vsx
implementations. As the FI bit is non-sticky, one could think that
xscmpeqdp, a instruction the ISA states doesn't change FI bit, should
result in a cleared FI. However, this is not happening on hardware.
An investigation resulted in the following conclusion:
If the ISA does not list the FI bit as altered for a particular
instruction, then it should be kept as it was before the instruction.
QEMU is not following this behavior. Affected instructions include:
- xv* (all vsx-vector instructions);
- xscmp*, xsmax*, xsmin*;
- xstdivdp and similars;
(to identify the affected instructions, just search in the ISA for
the instructions that does not list FI in "Special Registers Altered")
Most instructions use the function do_float_check_status() to commit
changes in the inexact flag. So the fix is to add a parameter to it
that will control if the bit FI should be changed or not.
All users of do_float_check_status() are then modified to provide this
argument, controlling if that specific instruction changes bit FI or
not.
Some macro helpers are responsible for both instructions that change
and instructions that aren't suposed to change FI. This seems to always
overlap with the sfprf flag. So, reuse this flag for this purpose when
applicable.
Signed-off-by: Víctor Colombo<victor.colombo@eldorado.org.br>
---
v2:
- move the FI change from float_inexact_excp to do_float_check_status
- sfprf will be renamed to sfifprf in another patch, as suggested by
Richard
---
target/ppc/cpu.h | 2 +
target/ppc/fpu_helper.c | 122 +++++++++++++++++++++-------------------
2 files changed, 66 insertions(+), 58 deletions(-)