Fix #79595: zend_init_fpu() alters FPU precision

On startup, PHP deliberately changes the floating point control word to
enforce binary64 format for the calculations for best consistency
across platforms.  However, this is unnessary for x86_64 architectures,
because in this case SSE instructions are used by default, and there is
no good reason to pass `-mfpmath=i387` or such.

Therefore, we can skip the modification, which has the benefit that
system libraries are free to work in the mode of their liking.

Author: cmb69

NEWS

@@ -2,7 +2,8 @@ PHP                                                                        NEWS
 |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 ?? ??? 2020, PHP 7.4.8
 
-
+- Core:
+  . Fixed bug #79595 (zend_init_fpu() alters FPU precision). (cmb, Nikita)
 
 11 Jun 2020, PHP 7.4.7
 

Zend/zend_float.h

@@ -55,10 +55,9 @@ END_EXTERN_C()
  Implementation notes:
 
  x86_64:
-  - Since all x86_64 compilers use SSE by default, it is probably unnecessary
-    to use these macros there. We define them anyway since we are too lazy
-    to differentiate the architecture. Also, the compiler option -mfpmath=i387
-    justifies this decision.
+  - Since all x86_64 compilers use SSE by default, we do not define these
+    macros there. We ignore the compiler option -mfpmath=i387, because there is
+    no reason to use it on x86_64.
 
  General:
   - It would be nice if one could detect whether SSE if used for math via some
@@ -67,17 +66,15 @@ END_EXTERN_C()
 
  MS Visual C:
   - Since MSVC users typically don't use autoconf or CMake, we will detect
-    MSVC via compile time define. Floating point precision change isn't
-    supported on 64 bit platforms, so it's NOP. See
-    http://msdn.microsoft.com/en-us/library/c9676k6h(v=vs.110).aspx
+    MSVC via compile time define.
 */
 
 /* MSVC detection (MSVC people usually don't use autoconf) */
 #if defined(_MSC_VER) && !defined(_WIN64)
 #  define HAVE__CONTROLFP_S
 #endif /* _MSC_VER */
 
-#ifdef HAVE__CONTROLFP_S
+#if defined(HAVE__CONTROLFP_S) && !defined(__x86_64__)
 
 /* float.h defines _controlfp_s */
 # include <float.h>
@@ -141,7 +138,7 @@ END_EXTERN_C()
                 return _xpfpa_result; \
             } while (0)
 
-#elif defined(HAVE__CONTROLFP)
+#elif defined(HAVE__CONTROLFP) && !defined(__x86_64__)
 
 /* float.h defines _controlfp */
 # include <float.h>
@@ -200,7 +197,7 @@ END_EXTERN_C()
                 return _xpfpa_result; \
             } while (0)
 
-#elif defined(HAVE__FPU_SETCW) /* glibc systems */
+#elif defined(HAVE__FPU_SETCW)  && !defined(__x86_64__) /* glibc systems */
 
 /* fpu_control.h defines _FPU_[GS]ETCW */
 # include <fpu_control.h>
@@ -259,7 +256,7 @@ END_EXTERN_C()
                 return _xpfpa_result; \
             } while (0)
 
-#elif defined(HAVE_FPSETPREC) /* FreeBSD */
+#elif defined(HAVE_FPSETPREC)  && !defined(__x86_64__) /* FreeBSD */
 
 /* fpu_control.h defines _FPU_[GS]ETCW */
 # include <machine/ieeefp.h>
@@ -315,7 +312,7 @@ END_EXTERN_C()
                 return _xpfpa_result; \
             } while (0)
 
-#elif defined(HAVE_FPU_INLINE_ASM_X86)
+#elif defined(HAVE_FPU_INLINE_ASM_X86) && !defined(__x86_64__)
 
 /*
   Custom x86 inline assembler implementation.