Re: [Mingw-w64-public] [PATCH 3/3] crt: Fix x86 __mingw_setfp() to not trigger exception when disabling x87 exceptions

[LIU Hao]

在 2025-12-28 02:32, Pali Rohár 写道:

x87 fldcw instruction triggers pending x87 exception. So if there is a
pending x87 exception and __mingw_setfp() is trying to disable some other
x87 type then it will trigger pending exception.

There is no non-waiting variant of x87 fldcw instruction. So for setting
new control word use x87 fldenv instruction like for setting new status
word. For using x87 fldenv instruction it is first need to load whole
x87 env via x87 fnstenv instruction.

Optimize code to not use x87 fnstenv when it is known that x87 fldenv would
not be used - when cw_mask or sw_mask is empty (used just for quering cw/sw).
---
  mingw-w64-crt/misc/mingw_setfp.c | 63 +++++++++++++++++++-------------
  1 file changed, 38 insertions(+), 25 deletions(-)

diff --git a/mingw-w64-crt/misc/mingw_setfp.c b/mingw-w64-crt/misc/mingw_setfp.c
index fb03781292ce..2298515128b5 100644
--- a/mingw-w64-crt/misc/mingw_setfp.c
+++ b/mingw-w64-crt/misc/mingw_setfp.c
...
+    if ((!sw || sw_mask == 0) && (!cw || cw_mask == 0))
+    {
+        /* Fast path: when we are not going to change sw/cw which is indicated
+         * by zero mask then load sw/cw via fast fnstsw/fnstcw instruction.
+         */
+        if (sw && sw_mask == 0)
+            __asm__ __volatile__( "fnstsw %0" : "=m" (newsw) );
+        if (cw && cw_mask == 0)
+            __asm__ __volatile__( "fnstcw %0" : "=m" (newcw) );
+    }
+    else
+    {
+        /* Slow path: when we are going to change sw/cw or we do not know yet 
then
+         * load whole x87 env via slow fnstenv as it is needed for changing 
sw/cw.
+         */
+        __asm__ __volatile__( "fnstenv %0" : "=m" (fenv) );
+        newsw = fenv.status_word;
+        newcw = fenv.control_word;
+    }
+
      if (sw)
      {
-        __asm__ __volatile__( "fnstsw %0" : "=m" (newsw) );
          oldsw = newsw;

At a first glance this doesn't look quite correct: When `sw` is null, `cw` is non-null and `cw_mask` is non-zero, this takes the slow path, and leaves `newsw` as zero, and eventually zeroes the status word.

I'm testing this series of patches with this change for now:


diff --git a/mingw-w64-crt/misc/mingw_setfp.c b/mingw-w64-crt/misc/mingw_setfp.c
index 229851512..dcd767d9a 100644
--- a/mingw-w64-crt/misc/mingw_setfp.c
+++ b/mingw-w64-crt/misc/mingw_setfp.c
@@ -144,10 +144,8 @@ void __mingw_setfp( unsigned int *cw, unsigned int cw_mask,
         /* Fast path: when we are not going to change sw/cw which is indicated
          * by zero mask then load sw/cw via fast fnstsw/fnstcw instruction.
          */
-        if (sw && sw_mask == 0)
-            __asm__ __volatile__( "fnstsw %0" : "=m" (newsw) );
-        if (cw && cw_mask == 0)
-            __asm__ __volatile__( "fnstcw %0" : "=m" (newcw) );
+        __asm__ __volatile__( "fnstsw %0" : "=m" (newsw) );
+        __asm__ __volatile__( "fnstcw %0" : "=m" (newcw) );
     }
     else
     {
@@ -159,10 +157,11 @@ void __mingw_setfp( unsigned int *cw, unsigned int 
cw_mask,
         newcw = fenv.control_word;
     }

+    oldsw = newsw;
+    oldcw = newcw;
+
     if (sw)
     {
-        oldsw = newsw;
-
         flags = 0;
         if (newsw & 0x1) flags |= _SW_INVALID;
         if (newsw & 0x2) flags |= _SW_DENORMAL;
@@ -184,8 +183,6 @@ void __mingw_setfp( unsigned int *cw, unsigned int cw_mask,

     if (cw)
     {
-        oldcw = newcw;
-
         flags = 0;
         if (newcw & 0x1) flags |= _EM_INVALID;
         if (newcw & 0x2) flags |= _EM_DENORMAL;




--
Best regards,
LIU Hao

Since the use of FLDENV is subject to a change in either word, it's necessary to initialize {old,new}{cw,sw} unconditionally.

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