According to Intel's x87 FPU specification (Table 8-10, Vol. 1), arithmetic
operations on operands in unsupported formats (including pseudo-NaNs) must
return the QNaN floating-point indefinite value.

The helper functions for FPATAN, FYL2XP1, and FYL2X incorrectly check for
signaling NaN before checking for invalid encodings. This causes pseudo-NaNs
to be treated as valid signaling NaNs and silenced, rather than being
rejected as unsupported formats.

Reorder the checks to test floatx80_invalid_encoding before
floatx80_is_signaling_nan, matching the correct behavior already
implemented in helper_fscale.

Signed-off-by: Max Chou <max.chou@sifive.com>
---
 target/i386/tcg/fpu_helper.c | 30 +++++++++++++++---------------
 1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/target/i386/tcg/fpu_helper.c b/target/i386/tcg/fpu_helper.c
index b3b23823fd..37c83ded38 100644
--- a/target/i386/tcg/fpu_helper.c
+++ b/target/i386/tcg/fpu_helper.c
@@ -1377,16 +1377,16 @@ void helper_fpatan(CPUX86State *env)
     int32_t arg1_exp = extractFloatx80Exp(ST1);
     bool arg1_sign = extractFloatx80Sign(ST1);
 
-    if (floatx80_is_signaling_nan(ST0, &env->fp_status)) {
+    if (floatx80_invalid_encoding(ST0, &env->fp_status) ||
+        floatx80_invalid_encoding(ST1, &env->fp_status)) {
+        float_raise(float_flag_invalid, &env->fp_status);
+        ST1 = floatx80_default_nan(&env->fp_status);
+    } else if (floatx80_is_signaling_nan(ST0, &env->fp_status)) {
         float_raise(float_flag_invalid, &env->fp_status);
         ST1 = floatx80_silence_nan(ST0, &env->fp_status);
     } else if (floatx80_is_signaling_nan(ST1, &env->fp_status)) {
         float_raise(float_flag_invalid, &env->fp_status);
         ST1 = floatx80_silence_nan(ST1, &env->fp_status);
-    } else if (floatx80_invalid_encoding(ST0, &env->fp_status) ||
-               floatx80_invalid_encoding(ST1, &env->fp_status)) {
-        float_raise(float_flag_invalid, &env->fp_status);
-        ST1 = floatx80_default_nan(&env->fp_status);
     } else if (floatx80_is_any_nan(ST0)) {
         ST1 = ST0;
     } else if (floatx80_is_any_nan(ST1)) {
@@ -2061,16 +2061,16 @@ void helper_fyl2xp1(CPUX86State *env)
     int32_t arg1_exp = extractFloatx80Exp(ST1);
     bool arg1_sign = extractFloatx80Sign(ST1);
 
-    if (floatx80_is_signaling_nan(ST0, &env->fp_status)) {
+    if (floatx80_invalid_encoding(ST0, &env->fp_status) ||
+        floatx80_invalid_encoding(ST1, &env->fp_status)) {
+        float_raise(float_flag_invalid, &env->fp_status);
+        ST1 = floatx80_default_nan(&env->fp_status);
+    } else if (floatx80_is_signaling_nan(ST0, &env->fp_status)) {
         float_raise(float_flag_invalid, &env->fp_status);
         ST1 = floatx80_silence_nan(ST0, &env->fp_status);
     } else if (floatx80_is_signaling_nan(ST1, &env->fp_status)) {
         float_raise(float_flag_invalid, &env->fp_status);
         ST1 = floatx80_silence_nan(ST1, &env->fp_status);
-    } else if (floatx80_invalid_encoding(ST0, &env->fp_status) ||
-               floatx80_invalid_encoding(ST1, &env->fp_status)) {
-        float_raise(float_flag_invalid, &env->fp_status);
-        ST1 = floatx80_default_nan(&env->fp_status);
     } else if (floatx80_is_any_nan(ST0)) {
         ST1 = ST0;
     } else if (floatx80_is_any_nan(ST1)) {
@@ -2159,16 +2159,16 @@ void helper_fyl2x(CPUX86State *env)
     int32_t arg1_exp = extractFloatx80Exp(ST1);
     bool arg1_sign = extractFloatx80Sign(ST1);
 
-    if (floatx80_is_signaling_nan(ST0, &env->fp_status)) {
+    if (floatx80_invalid_encoding(ST0, &env->fp_status) ||
+        floatx80_invalid_encoding(ST1, &env->fp_status)) {
+        float_raise(float_flag_invalid, &env->fp_status);
+        ST1 = floatx80_default_nan(&env->fp_status);
+    } else if (floatx80_is_signaling_nan(ST0, &env->fp_status)) {
         float_raise(float_flag_invalid, &env->fp_status);
         ST1 = floatx80_silence_nan(ST0, &env->fp_status);
     } else if (floatx80_is_signaling_nan(ST1, &env->fp_status)) {
         float_raise(float_flag_invalid, &env->fp_status);
         ST1 = floatx80_silence_nan(ST1, &env->fp_status);
-    } else if (floatx80_invalid_encoding(ST0, &env->fp_status) ||
-               floatx80_invalid_encoding(ST1, &env->fp_status)) {
-        float_raise(float_flag_invalid, &env->fp_status);
-        ST1 = floatx80_default_nan(&env->fp_status);
     } else if (floatx80_is_any_nan(ST0)) {
         ST1 = ST0;
     } else if (floatx80_is_any_nan(ST1)) {
-- 
2.52.0

On 2/4/26 15:17, Max Chou wrote:
> According to Intel's x87 FPU specification (Table 8-10, Vol. 1), arithmetic
> operations on operands in unsupported formats (including pseudo-NaNs) must
> return the QNaN floating-point indefinite value.
> 
> The helper functions for FPATAN, FYL2XP1, and FYL2X incorrectly check for
> signaling NaN before checking for invalid encodings. This causes pseudo-NaNs
> to be treated as valid signaling NaNs and silenced, rather than being
> rejected as unsupported formats.
> 
> Reorder the checks to test floatx80_invalid_encoding before
> floatx80_is_signaling_nan, matching the correct behavior already
> implemented in helper_fscale.
> 
> Signed-off-by: Max Chou<max.chou@sifive.com>
> ---
>   target/i386/tcg/fpu_helper.c | 30 +++++++++++++++---------------
>   1 file changed, 15 insertions(+), 15 deletions(-)

Good catch.

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


r~