[v3,2/3] target/ppc: Fix FPSCR.FI changing in float_overflow_excp()

Commit Message

Víctor Colombo May 17, 2022, 4:15 p.m. UTC

This patch fixes another not-so-clear situation in Power ISA
regarding the inexact bits in FPSCR. The ISA states that:

"""
When Overflow Exception is disabled (OE=0) and an
Overflow Exception occurs, the following actions are
taken:
...
2. Inexact Exception is set
XX <- 1
...
FI is set to 1
...
"""

However, when tested on a Power 9 hardware, some instructions that
trigger an OX don't set the FI bit:

xvcvdpsp(0x4050533fcdb7b95ff8d561c40bf90996) = FI: CLEARED -> CLEARED
xvnmsubmsp(0xf3c0c1fc8f3230, 0xbeaab9c5) = FI: CLEARED -> CLEARED
(just a few examples. Other instructions are also affected)

The root cause for this seems to be that only instructions that list
the bit FI in the "Special Registers Altered" should modify it.

QEMU is, today, not working like the hardware:

xvcvdpsp(0x4050533fcdb7b95ff8d561c40bf90996) = FI: CLEARED -> SET
xvnmsubmsp(0xf3c0c1fc8f3230, 0xbeaab9c5) = FI: CLEARED -> SET

(all tests assume FI is cleared beforehand)

Fix this by making float_overflow_excp() return float_flag_inexact
if it should update the inexact flags.

Signed-off-by: Víctor Colombo <[e-mail]>
Reviewed-by: Richard Henderson <[e-mail]>
Reviewed-by: Rashmica Gupta <[e-mail]>
---
 target/ppc/fpu_helper.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

Patch

diff --git a/target/ppc/fpu_helper.c b/target/ppc/fpu_helper.c
index f1ea4aa10e..88f9e756a5 100644
--- a/target/ppc/fpu_helper.c
+++ b/target/ppc/fpu_helper.c
@@ -329,24 +329,25 @@  static inline void float_zero_divide_excp(CPUPPCState *env, uintptr_t raddr)
     }
 }
 
-static inline void float_overflow_excp(CPUPPCState *env)
+static inline int float_overflow_excp(CPUPPCState *env)
 {
     CPUState *cs = env_cpu(env);
 
     env->fpscr |= FP_OX;
     /* Update the floating-point exception summary */
     env->fpscr |= FP_FX;
-    if (env->fpscr & FP_OE) {
+
+    bool overflow_enabled = !!(env->fpscr & FP_OE);
+    if (overflow_enabled) {
         /* XXX: should adjust the result */
         /* Update the floating-point enabled exception summary */
         env->fpscr |= FP_FEX;
         /* We must update the target FPR before raising the exception */
         cs->exception_index = POWERPC_EXCP_PROGRAM;
         env->error_code = POWERPC_EXCP_FP | POWERPC_EXCP_FP_OX;
-    } else {
-        env->fpscr |= FP_XX;
-        env->fpscr |= FP_FI;
     }
+
+    return overflow_enabled ? 0 : float_flag_inexact;
 }
 
 static inline void float_underflow_excp(CPUPPCState *env)
@@ -468,7 +469,7 @@  static void do_float_check_status(CPUPPCState *env, bool change_fi,
     int status = get_float_exception_flags(&env->fp_status);
 
     if (status & float_flag_overflow) {
-        float_overflow_excp(env);
+        status |= float_overflow_excp(env);
     } else if (status & float_flag_underflow) {
         float_underflow_excp(env);
     }