[PATCH v2 2/2] ntdll: Avoid resetting x87 FPU state when saving context.

[Paul Gofman]

'FNSAVE' x87 instruction resets the FPU state to default values
(like FINIT). This results in FPU state reset to default in any
ntdll call which is getting x87 FPU state, e. g. obtaining context
or raising an exception.

Signed-off-by: Paul Gofman 
---
v2: no changes.

dlls/ntdll/signal_i386.c     | 18 ++++++++++++++++++
 dlls/ntdll/tests/exception.c |  2 +-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/dlls/ntdll/signal_i386.c b/dlls/ntdll/signal_i386.c
index 0274a7d26d..f1deb1c65c 100644
--- a/dlls/ntdll/signal_i386.c
+++ b/dlls/ntdll/signal_i386.c
@@ -860,8 +860,26 @@ static inline void *init_handler( const ucontext_t *sigcontext, WORD *fs, WORD *
 static inline void save_fpu( CONTEXT *context )
 {
 #ifdef __GNUC__
+    struct
+    {
+        DWORD ControlWord;
+        DWORD StatusWord;
+        DWORD TagWord;
+        DWORD ErrorOffset;
+        DWORD ErrorSelector;
+        DWORD DataOffset;
+        DWORD DataSelector;
+    }
+    float_status;
+
     context->ContextFlags |= CONTEXT_FLOATING_POINT;
     __asm__ __volatile__( "fnsave %0; fwait" : "=m" (context->FloatSave) );
+
+    /* Reset unmasked exceptions status to avoid firing an exception. */
+    memcpy(&float_status, &context->FloatSave, sizeof(float_status));
+    float_status.StatusWord &= float_status.ControlWord | 0xffffff80;
+
+    __asm__ __volatile__( "fldenv %0" : "=m" (float_status) );
 #endif
 }

diff --git a/dlls/ntdll/tests/exception.c b/dlls/ntdll/tests/exception.c
index f1a59f1dac..57ad30f180 100644
--- a/dlls/ntdll/tests/exception.c
+++ b/dlls/ntdll/tests/exception.c
@@ -1642,7 +1642,7 @@ static void test_thread_context(void)

ok( LOWORD(context.FloatSave.ControlWord) == LOWORD(expect.x87_control),
             "wrong x87 control word %#x/%#x.\n", context.FloatSave.ControlWord, expect.x87_control );
-    todo_wine ok( LOWORD(expect.x87_control) == LOWORD(new_x87_control),
+    ok( LOWORD(expect.x87_control) == LOWORD(new_x87_control),
             "x87 control word changed in NtGetContextThread() %#x/%#x.\n",
             LOWORD(expect.x87_control), LOWORD(new_x87_control) );