target/i386: Update FPU tag word for FXTRACT's old ST(0)

[PATCH] target/i386: Update FPU tag word for FXTRACT's old ST(0)

Simon Scherer posted 1 patch 3 weeks, 4 days ago

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git fetch https://github.com/patchew-project/qemu tags/patchew/20260901085839.138094-1-scherer.simon89@gmail.com

Maintainers: Paolo Bonzini <pbonzini@redhat.com>, Richard Henderson <richard.henderson@linaro.org>

target/i386/tcg/fpu_helper.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)

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[PATCH] target/i386: Update FPU tag word for FXTRACT's old ST(0)

Posted by Simon Scherer 3 weeks, 4 days ago

FXTRACT writes two registers: the old ST(0) gets the exponent (it
becomes ST(1) after the push), and the new ST(0) gets the
significand. Both should end up marked valid in the FPU tag word.

helper_fxtract() only marks the new one valid, via fpush(). The old
ST(0) just keeps whatever tag it had before the instruction, even
though it was just written with the exponent. So if it happened to
be tagged empty beforehand, it is still (wrongly) tagged empty
afterwards.

Add a fpush_fxtract() helper that does the usual fpush() and also
marks the old ST(0) (now ST(1)) valid, and use it in place of plain
fpush() at all previous call sites in helper_fxtract().

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4395
Signed-off-by: Simon Scherer <scherer.simon89@gmail.com>
---
 target/i386/tcg/fpu_helper.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/target/i386/tcg/fpu_helper.c b/target/i386/tcg/fpu_helper.c
index b812125efa..c17ab7282b 100644
--- a/target/i386/tcg/fpu_helper.c
+++ b/target/i386/tcg/fpu_helper.c
@@ -1805,6 +1805,13 @@ void helper_fpatan(CPUX86State *env)
     merge_exception_flags(env, old_flags);
 }
 
+/* fpush() only validates the new top. FXTRACT also needs ST(1) validated. */
+static inline void fpush_fxtract(CPUX86State *env)
+{
+    fpush(env);
+    env->fptags[(env->fpstt + 1) & 7] = 0;
+}
+
 void helper_fxtract(CPUX86State *env)
 {
     int old_flags = save_exception_flags(env);
@@ -1816,22 +1823,22 @@ void helper_fxtract(CPUX86State *env)
         /* Easy way to generate -inf and raising division by 0 exception */
         ST0 = floatx80_div(floatx80_chs(floatx80_one), floatx80_zero,
                            &env->fp_status);
-        fpush(env);
+        fpush_fxtract(env);
         ST0 = temp.d;
     } else if (floatx80_invalid_encoding(ST0, &env->fp_status)) {
         float_raise(float_flag_invalid, &env->fp_status);
         ST0 = floatx80_default_nan(&env->fp_status);
-        fpush(env);
+        fpush_fxtract(env);
         ST0 = ST1;
     } else if (floatx80_is_any_nan(ST0)) {
         if (floatx80_is_signaling_nan(ST0, &env->fp_status)) {
             float_raise(float_flag_invalid, &env->fp_status);
             ST0 = floatx80_silence_nan(ST0, &env->fp_status);
         }
-        fpush(env);
+        fpush_fxtract(env);
         ST0 = ST1;
     } else if (floatx80_is_infinity(ST0, &env->fp_status)) {
-        fpush(env);
+        fpush_fxtract(env);
         ST0 = ST1;
         ST1 = floatx80_default_inf(0, &env->fp_status);
     } else {
@@ -1847,7 +1854,7 @@ void helper_fxtract(CPUX86State *env)
         }
         /* DP exponent bias */
         ST0 = int32_to_floatx80(expdif, &env->fp_status);
-        fpush(env);
+        fpush_fxtract(env);
         BIASEXPONENT(temp);
         ST0 = temp.d;
     }
-- 
2.53.0