[v2,for-10.0,40/54] target/arm: Set default NaN pattern explicitly

Commit Message

Peter Maydell Dec. 2, 2024, 1:13 p.m. UTC

Set the default NaN pattern explicitly for the arm target.
This includes setting it for the old linux-user nwfpe emulation.
For nwfpe, our default doesn't match the real kernel, but we
avoid making a behaviour change in this commit.

Signed-off-by: Peter Maydell <[e-mail]>
---
 linux-user/arm/nwfpe/fpa11.c | 5 +++++
 target/arm/cpu.c             | 2 ++
 2 files changed, 7 insertions(+)

Comments

Richard Henderson Dec. 2, 2024, 5:29 p.m. UTC | #1

On 12/2/24 07:13, Peter Maydell wrote:
> Set the default NaN pattern explicitly for the arm target.
> This includes setting it for the old linux-user nwfpe emulation.
> For nwfpe, our default doesn't match the real kernel, but we
> avoid making a behaviour change in this commit.
> 
> Signed-off-by: Peter Maydell <[e-mail]>
> ---
>   linux-user/arm/nwfpe/fpa11.c | 5 +++++
>   target/arm/cpu.c             | 2 ++
>   2 files changed, 7 insertions(+)

Reviewed-by: Richard Henderson <[e-mail]>

r~

Patch

diff --git a/linux-user/arm/nwfpe/fpa11.c b/linux-user/arm/nwfpe/fpa11.c
index 8356beb52c6..0f1afbd91df 100644
--- a/linux-user/arm/nwfpe/fpa11.c
+++ b/linux-user/arm/nwfpe/fpa11.c
@@ -69,6 +69,11 @@  void resetFPA11(void)
    * this late date.
    */
   set_float_2nan_prop_rule(float_2nan_prop_s_ab, &fpa11->fp_status);
+  /*
+   * Use the same default NaN value as Arm VFP. This doesn't match
+   * the Linux kernel's nwfpe emulation, which uses an all-1s value.
+   */
+  set_float_default_nan_pattern(0b01000000, &fpa11->fp_status);
 }
 
 void SetRoundingMode(const unsigned int opcode)
diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index c81f6df3fca..4f7e18eb8e6 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -179,6 +179,7 @@  void arm_register_el_change_hook(ARMCPU *cpu, ARMELChangeHookFn *hook,
  *    the pseudocode function the arguments are in the order c, a, b.
  *  * 0 * Inf + NaN returns the default NaN if the input NaN is quiet,
  *    and the input NaN if it is signalling
+ *  * Default NaN has sign bit clear, msb frac bit set
  */
 static void arm_set_default_fp_behaviours(float_status *s)
 {
@@ -186,6 +187,7 @@  static void arm_set_default_fp_behaviours(float_status *s)
     set_float_2nan_prop_rule(float_2nan_prop_s_ab, s);
     set_float_3nan_prop_rule(float_3nan_prop_s_cab, s);
     set_float_infzeronan_rule(float_infzeronan_dnan_if_qnan, s);
+    set_float_default_nan_pattern(0b01000000, s);
 }
 
 static void cp_reg_reset(gpointer key, gpointer value, gpointer opaque)