FPCR.AH == 1 mandates that taking the absolute value of a NaN should
not change its sign bit.  This means we can no longer use
gen_vfp_abs*() everywhere but must instead generate slightly more
complex code when FPCR.AH is set.

Implement these semantics for scalar FABS and FABD.  This change also
affects all other instructions whose psuedocode calls FPAbs(); we
will extend the change to those instructions in following commits.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/tcg/translate-a64.c | 69 +++++++++++++++++++++++++++++++++-
 1 file changed, 67 insertions(+), 2 deletions(-)

diff --git a/target/arm/tcg/translate-a64.c b/target/arm/tcg/translate-a64.c
index 085b29ee536..542e774790b 100644
--- a/target/arm/tcg/translate-a64.c
+++ b/target/arm/tcg/translate-a64.c
@@ -880,6 +880,43 @@ static void gen_vfp_ah_negd(TCGv_i64 d, TCGv_i64 s)
                         s, chs_s);
 }
 
+/*
+ * These functions implement
+ *  d = floatN_is_any_nan(s) ? s : floatN_abs(s)
+ * which for float32 is
+ *  d = (s & ~(1 << 31)) > 0x7f800000UL) ? s : (s & ~(1 << 31))
+ * and similarly for the other float sizes.
+ */
+static void gen_vfp_ah_absh(TCGv_i32 d, TCGv_i32 s)
+{
+    TCGv_i32 abs_s = tcg_temp_new_i32();
+
+    gen_vfp_absh(abs_s, s);
+    tcg_gen_movcond_i32(TCG_COND_GTU, d,
+                        abs_s, tcg_constant_i32(0x7c00),
+                        s, abs_s);
+}
+
+static void gen_vfp_ah_abss(TCGv_i32 d, TCGv_i32 s)
+{
+    TCGv_i32 abs_s = tcg_temp_new_i32();
+
+    gen_vfp_abss(abs_s, s);
+    tcg_gen_movcond_i32(TCG_COND_GTU, d,
+                        abs_s, tcg_constant_i32(0x7f800000UL),
+                        s, abs_s);
+}
+
+static void gen_vfp_ah_absd(TCGv_i64 d, TCGv_i64 s)
+{
+    TCGv_i64 abs_s = tcg_temp_new_i64();
+
+    gen_vfp_absd(abs_s, s);
+    tcg_gen_movcond_i64(TCG_COND_GTU, d,
+                        abs_s, tcg_constant_i64(0x7ff0000000000000ULL),
+                        s, abs_s);
+}
+
 static void gen_vfp_maybe_ah_negh(DisasContext *dc, TCGv_i32 d, TCGv_i32 s)
 {
     if (dc->fpcr_ah) {
@@ -5403,12 +5440,35 @@ static void gen_fabd_d(TCGv_i64 d, TCGv_i64 n, TCGv_i64 m, TCGv_ptr s)
     gen_vfp_absd(d, d);
 }
 
+static void gen_fabd_ah_h(TCGv_i32 d, TCGv_i32 n, TCGv_i32 m, TCGv_ptr s)
+{
+    gen_helper_vfp_subh(d, n, m, s);
+    gen_vfp_ah_absh(d, d);
+}
+
+static void gen_fabd_ah_s(TCGv_i32 d, TCGv_i32 n, TCGv_i32 m, TCGv_ptr s)
+{
+    gen_helper_vfp_subs(d, n, m, s);
+    gen_vfp_ah_abss(d, d);
+}
+
+static void gen_fabd_ah_d(TCGv_i64 d, TCGv_i64 n, TCGv_i64 m, TCGv_ptr s)
+{
+    gen_helper_vfp_subd(d, n, m, s);
+    gen_vfp_ah_absd(d, d);
+}
+
 static const FPScalar f_scalar_fabd = {
     gen_fabd_h,
     gen_fabd_s,
     gen_fabd_d,
 };
-TRANS(FABD_s, do_fp3_scalar, a, &f_scalar_fabd, a->rn)
+static const FPScalar f_scalar_ah_fabd = {
+    gen_fabd_ah_h,
+    gen_fabd_ah_s,
+    gen_fabd_ah_d,
+};
+TRANS(FABD_s, do_fp3_scalar_2fn, a, &f_scalar_fabd, &f_scalar_ah_fabd, a->rn)
 
 static const FPScalar f_scalar_frecps = {
     gen_helper_recpsf_f16,
@@ -8654,7 +8714,12 @@ static const FPScalar1Int f_scalar_fabs = {
     gen_vfp_abss,
     gen_vfp_absd,
 };
-TRANS(FABS_s, do_fp1_scalar_int, a, &f_scalar_fabs, true)
+static const FPScalar1Int f_scalar_ah_fabs = {
+    gen_vfp_ah_absh,
+    gen_vfp_ah_abss,
+    gen_vfp_ah_absd,
+};
+TRANS(FABS_s, do_fp1_scalar_int_2fn, a, &f_scalar_fabs, &f_scalar_ah_fabs)
 
 static const FPScalar1Int f_scalar_fneg = {
     gen_vfp_negh,
-- 
2.34.1

On 1/24/25 08:28, Peter Maydell wrote:
> FPCR.AH == 1 mandates that taking the absolute value of a NaN should
> not change its sign bit.  This means we can no longer use
> gen_vfp_abs*() everywhere but must instead generate slightly more
> complex code when FPCR.AH is set.
> 
> Implement these semantics for scalar FABS and FABD.  This change also
> affects all other instructions whose psuedocode calls FPAbs(); we
> will extend the change to those instructions in following commits.
> 
> Signed-off-by: Peter Maydell<peter.maydell@linaro.org>
> ---
>   target/arm/tcg/translate-a64.c | 69 +++++++++++++++++++++++++++++++++-
>   1 file changed, 67 insertions(+), 2 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~