[Qemu-devel] [PATCH] target-ppc: Bug: VSX Convert to Integer Should Truncate

[Tom Musta <tommusta@gmail.com>]

The various VSX Convert to Integer instructions should truncate the
mantissa.  This fix forces the softfloat rounding mode to "round to
zero" prior to performing the conversion.  After the conversion is
completed, the internal rounding mode is restored from the PowerPC
FPSCR bits.

Signed-off-by: Tom Musta <tommusta@gmail.com>
---
This bug was discovered when running wget, which does this:

    double maxtime;
    struct timeval tmout;
    ...
    tmout.tv_usec = 1000000 * (maxtime - (long) maxtime);

The newest PowerPC 64-bit gcc's are now using xscvdpsxds to perform the cast of 
the double to long.  A timeout of 0.95 was erroneously rounding up to 1 and
hence computing a negative timeout value.

It would be great if we could still get this into 2.0.
  
 target-ppc/fpu_helper.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/target-ppc/fpu_helper.c b/target-ppc/fpu_helper.c
index e7f3295..ccfc5cc 100644
--- a/target-ppc/fpu_helper.c
+++ b/target-ppc/fpu_helper.c
@@ -2558,10 +2558,14 @@ void helper_##op(CPUPPCState *env, uint32_t opcode)                          \
             fload_invalid_op_excp(env, POWERPC_EXCP_FP_VXCVI, 0);            \
             xt.tfld = rnan;                                                  \
         } else {                                                             \
+            /* force round to zero mode (truncation) */                      \
+            set_float_rounding_mode(float_round_to_zero, &env->fp_status);   \
             xt.tfld = stp##_to_##ttp(xb.sfld, &env->fp_status);              \
             if (env->fp_status.float_exception_flags & float_flag_invalid) { \
                 fload_invalid_op_excp(env, POWERPC_EXCP_FP_VXCVI, 0);        \
             }                                                                \
+            /* restore rounding mode from FPSCR */                           \
+            fpscr_set_rounding_mode(env);                                    \
         }                                                                    \
     }                                                                        \
                                                                              \
-- 
1.7.1