[PATCH 1/2] target/i386: Update FPU tag word for FSTP

[Simon Scherer <scherer.simon89@gmail.com>]

FSTP ST(i) copies the value in ST(0) into ST(i) and then pops the
register stack. ST(i) should end up marked valid in the FPU tag word
and the old ST(0) should end up marked empty.

helper_fmov_STN_ST0() copies the value into ST(i) but never touches
its tag. So if ST(i) happened to be tagged empty beforehand, it is
still wrongly tagged empty afterwards.

Mark the destination tag to valid in helper_fmov_STN_ST0().
This also fixes FST ST(i), which shares the same helper.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4400
Signed-off-by: Simon Scherer <scherer.simon89@gmail.com>
---
 target/i386/tcg/fpu_helper.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/target/i386/tcg/fpu_helper.c b/target/i386/tcg/fpu_helper.c
index b812125efa..6ccc3c3b4e 100644
--- a/target/i386/tcg/fpu_helper.c
+++ b/target/i386/tcg/fpu_helper.c
@@ -510,6 +510,7 @@ void helper_fmov_ST0_STN(CPUX86State *env, int st_index)
 void helper_fmov_STN_ST0(CPUX86State *env, int st_index)
 {
     ST(st_index) = ST0;
+    env->fptags[(env->fpstt + st_index) & 7] = 0;
 }
 
 void helper_fxchg_ST0_STN(CPUX86State *env, int st_index)
-- 
2.53.0