Disable MP_SMALL_STACK_SIZE on MSVC

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

appveyor.yml

@@ -14,13 +14,7 @@ image:
 environment:
   matrix:
   - CFLAGS_VAR: ""
-    CFLAGS_VAR_DLL: "CFLAGS=\"/Ox /Oi /MD /DLTM_TEST_DYNAMIC\""
-
-  - CFLAGS_VAR: "CFLAGS=\"/Ox /Oi /DMP_SMALL_STACK_SIZE\""
-    CFLAGS_VAR_DLL: "CFLAGS=\"/Ox /Oi /MD /DLTM_TEST_DYNAMIC /DMP_SMALL_STACK_SIZE\""
-
-  - CFLAGS_VAR: "CFLAGS=\"/Ox /Oi /DMP_SMALL_STACK_SIZE /DLTM_TEST_MULTITHREAD\""
-    CFLAGS_VAR_DLL: "CFLAGS=\"/Ox /Oi /MD /DLTM_TEST_DYNAMIC /DMP_SMALL_STACK_SIZE /DLTM_TEST_MULTITHREAD\""
+    CFLAGS_VAR_DLL: "CFLAGS=\"/Ox /MD /DLTM_TEST_DYNAMIC\""
 build_script:
 - cmd: >-
     if "Visual Studio 2022"=="%APPVEYOR_BUILD_WORKER_IMAGE%" call "C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Auxiliary\Build\vcvars64.bat"

makefile.msvc

@@ -11,7 +11,7 @@
 
 #The following can be overridden from command line e.g. make -f makefile.msvc CC=gcc ARFLAGS=rcs
 PREFIX    = c:\devel
-CFLAGS    = /Ox /Oi
+CFLAGS    = /Ox
 LDFLAGS   =
 
 #Compilation flags

tommath_private.h

@@ -235,6 +235,23 @@ MP_PRIVATE mp_err s_mp_fp_log(const mp_int *a, mp_int *c) MP_WUR;
 MP_PRIVATE mp_err s_mp_fp_log_d(const mp_int *a, mp_word *c) MP_WUR;
 
 #ifdef MP_SMALL_STACK_SIZE
+
+#if defined(__GNUC__)
+/* We use TLS (Thread Local Storage) to manage the instance of the WARRAY
+ * per thread.
+ * The compilers we're usually looking at are GCC, Clang and MSVC.
+ * Both GCC and Clang are straight-forward with TLS, so it's enabled there.
+ * Using MSVC the tests were OK with the static library, but failed when
+ * the library was built as a DLL. As a result we completely disable
+ * support for MSVC.
+ * If your compiler can handle TLS properly without too much hocus pocus,
+ * feel free to open a PR to add support for it.
+ */
+#define mp_thread __thread
+#else
+#error "MP_SMALL_STACK_SIZE not supported with your compiler"
+#endif
+
 #define MP_SMALL_STACK_SIZE_C
 #define MP_ALLOC_WARRAY(name) *name = s_mp_warray_get()
 #define MP_FREE_WARRAY(name) s_mp_warray_put(name)
@@ -245,10 +262,8 @@ MP_PRIVATE mp_err s_mp_fp_log_d(const mp_int *a, mp_word *c) MP_WUR;
 #define MP_CHECK_WARRAY(name)
 #endif
 
-#if defined(_MSC_VER)
-#define mp_thread __declspec(thread)
-#elif defined(__GNUC__)
-#define mp_thread __thread
+#ifndef mp_thread
+#define mp_thread
 #endif
 
 typedef struct {