target/arm: Fix 48-bit saturating shifts

In do_sqrshl48_d() and do_uqrshl48_d() we got some of the edge
cases wrong and failed to saturate correctly:

(1) In do_sqrshl48_d() we used the same code that do_shrshl_bhs()
does to obtain the saturated most-negative and most-positive 48-bit
signed values for the large-shift-left case.  This gives (1 << 47)
for saturate-to-most-negative, but we weren't sign-extending this
value to the 64-bit output as the pseudocode requires.

(2) For left shifts by less than 48, we copied the "8/16 bit" code
from do_sqrshl_bhs() and do_uqrshl_bhs().  This doesn't do the right
thing because it assumes the C type we're working with is at least
twice the number of bits we're saturating to (so that a shift left by
bits-1 can't shift anything off the top of the value).  This isn't
true for bits == 48, so we would incorrectly return 0 rather than the
most-positive value for situations like "shift (1 << 44) right by
20".  Instead check for saturation by doing the shift and signextend
and then testing whether shifting back left again gives the original
value.

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

0 files changed, 0 insertions, 0 deletions