The UMOPA/UMOPS instructions are supposed to multiply unsigned 8 or
16 bit elements and accumulate the products into a 64-bit element.
In the Arm ARM pseudocode, this is done with the usual
infinite-precision signed arithmetic. However our implementation
doesn't quite get it right, because in the DEF_IMOP_64() macro we do:
sum += (NTYPE)(n >> 0) * (MTYPE)(m >> 0);
where NTYPE and MTYPE are uint16_t or int16_t. In the uint16_t case,
the C usual arithmetic conversions mean the values are converted to
"int" type and the multiply is done as a 32-bit multiply. This means
that if the inputs are, for example, 0xffff and 0xffff then the
result is 0xFFFE0001 as an int, which is then promoted to uint64_t
for the accumulation into sum; this promotion incorrectly sign
extends the multiply.
Avoid the incorrect sign extension by casting to int64_t before
the multiply, so we do the multiply as 64-bit signed arithmetic,
which is a type large enough that the multiply can never
overflow into the sign bit.
(The equivalent 8-bit operations in DEF_IMOP_32() are fine, because
the 8-bit multiplies can never overflow into the sign bit of a
32-bit integer.)
Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2372
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240722172957.1041231-3-peter.maydell@linaro.org
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