gh-142342: Fix m68k assembler operand constraints for %fpcr access (gh-142343)

On m68k, an fmove instruction accessing %fpcr may only move from
or to a data register or a memory operand. The constraint "g" also
permits the use of address registers, which is invalid. The correct
constraint is "dm". Beginning with GCC 15, the register allocator
picks an address register in the code which causes SIGILL during
runtime.

Co-authored-by: Michael Karcher <[email protected]>

Author: glaubitz

Include/internal/pycore_pymath.h

@@ -146,17 +146,17 @@ extern void _Py_set_387controlword(unsigned short);
     unsigned int old_fpcr, new_fpcr
 #define _Py_SET_53BIT_PRECISION_START                                   \
     do {                                                                \
-        __asm__ ("fmove.l %%fpcr,%0" : "=g" (old_fpcr));                \
+        __asm__ ("fmove.l %%fpcr,%0" : "=dm" (old_fpcr));               \
         /* Set double precision / round to nearest.  */                 \
         new_fpcr = (old_fpcr & ~0xf0) | 0x80;                           \
         if (new_fpcr != old_fpcr) {                                     \
-              __asm__ volatile ("fmove.l %0,%%fpcr" : : "g" (new_fpcr));\
+            __asm__ volatile ("fmove.l %0,%%fpcr" : : "dm" (new_fpcr)); \
         }                                                               \
     } while (0)
 #define _Py_SET_53BIT_PRECISION_END                                     \
     do {                                                                \
         if (new_fpcr != old_fpcr) {                                     \
-            __asm__ volatile ("fmove.l %0,%%fpcr" : : "g" (old_fpcr));  \
+            __asm__ volatile ("fmove.l %0,%%fpcr" : : "dm" (old_fpcr)); \
         }                                                               \
     } while (0)
 #endif

Misc/NEWS.d/next/Core_and_Builtins/2025-12-08-13-04-37.gh-issue-142343.BTAyML.rst

@@ -0,0 +1 @@
+Fix SIGILL crash on m68k due to incorrect assembly constraint.

configure

@@ -6133,8 +6133,8 @@ AS_VAR_IF([ac_cv_gcc_asm_for_x87], [yes], [
 AC_CACHE_CHECK([whether we can use gcc inline assembler to get and set mc68881 fpcr], [ac_cv_gcc_asm_for_mc68881], [
 AC_LINK_IFELSE(   [AC_LANG_PROGRAM([[]], [[
   unsigned int fpcr;
-  __asm__ __volatile__ ("fmove.l %%fpcr,%0" : "=g" (fpcr));
-  __asm__ __volatile__ ("fmove.l %0,%%fpcr" : : "g" (fpcr));
+  __asm__ __volatile__ ("fmove.l %%fpcr,%0" : "=dm" (fpcr));
+  __asm__ __volatile__ ("fmove.l %0,%%fpcr" : : "dm" (fpcr));
 ]])],[ac_cv_gcc_asm_for_mc68881=yes],[ac_cv_gcc_asm_for_mc68881=no])
 ])
 AS_VAR_IF([ac_cv_gcc_asm_for_mc68881], [yes], [