target/arm: Fix SMLAD incorrect setting of Q bit

target/arm: Fix SMLAD incorrect setting of Q bit

The SMLAD instruction is supposed to:
 * signed multiply Rn[15:0] * Rm[15:0]
 * signed multiply Rn[31:16] * Rm[31:16]
 * perform a signed addition of the products and Ra
 * set Rd to the low 32 bits of the theoretical
   infinite-precision result
 * set the Q flag if the sign-extension of Rd
   would differ from the infinite-precision result
   (ie on overflow)

Our current implementation doesn't quite do this, though: it performs
an addition of the products setting Q on overflow, and then it adds
Ra, again possibly setting Q.  This sometimes incorrectly sets Q when
the architecturally mandated only-check-for-overflow-once algorithm
does not. For instance:
 r1 = 0x80008000; r2 = 0x80008000; r3 = 0xffffffff
 smlad r0, r1, r2, r3
This is (-32768 * -32768) + (-32768 * -32768) - 1

The products are both 0x4000_0000, so when added together as 32-bit
signed numbers they overflow (and QEMU sets Q), but because the
addition of Ra == -1 brings the total back down to 0x7fff_ffff
there is no overflow for the complete operation and setting Q is
incorrect.

Fix this edge case by resorting to 64-bit arithmetic for the
case where we need to add three values together.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201009144712.11187-1-peter.maydell@linaro.org