gen_context: Don't include library config files

Before this commit, gen_context.c both included libsecp256k1-config.h
and basic-config.h: The former only to obtain ECMULT_GEN_PREC_BITS
and the latter to obtain a basic working configuration to be able to
use the library.

This was inelegant and confusing: It meant that basic-config.h needs
to #undef all the macros defined in libsecp256k1-config.h. Moreover,
it meant that basic-config.h cannot define ECMULT_GEN_PREC_BITS,
essentially making this file specific for use in gen_context.c.

After this commit, gen_context.c does not include any configuration
file. Now, ECMULT_GEN_PREC_BITS is expected to be passed via the
command line and make will do this. Moreover, basic-config.h is not
necessary anymore for the modules used in gen_context.c because
79f1f7a made the preprocessor detect
all the relevant config options.

On the way, we remove an unused #define in basic-config.h.

Author: real-or-random

Makefile.am

@@ -128,8 +128,10 @@ CPPFLAGS_FOR_BUILD +=-I$(top_srcdir) -I$(builddir)/src
 
 gen_context_OBJECTS = gen_context.o
 gen_context_BIN = gen_context$(BUILD_EXEEXT)
-gen_%.o: src/gen_%.c src/libsecp256k1-config.h
-	$(CC_FOR_BUILD) $(CPPFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
+gen_context_CPPFLAGS = -DECMULT_GEN_PREC_BITS=$(ECMULT_GEN_PREC_BITS)
+
+$(gen_context_OBJECTS): src/gen_context.c
+	$(CC_FOR_BUILD) $(CPPFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) $(gen_context_CPPFLAGS) -c $< -o $@
 
 $(gen_context_BIN): $(gen_context_OBJECTS)
 	$(CC_FOR_BUILD) $(CFLAGS_FOR_BUILD) $(LDFLAGS_FOR_BUILD) $^ -o $@

configure.ac

@@ -315,6 +315,7 @@ fi
 case $set_ecmult_gen_precision in
 2|4|8)
   AC_DEFINE_UNQUOTED(ECMULT_GEN_PREC_BITS, $set_ecmult_gen_precision, [Set ecmult gen precision bits])
+  AC_SUBST(ECMULT_GEN_PREC_BITS, $set_ecmult_gen_precision)
   ;;
 *)
   AC_MSG_ERROR(['ecmult gen precision not 2, 4, 8 or "auto"'])

src/basic-config.h

@@ -9,15 +9,6 @@
 
 #ifdef USE_BASIC_CONFIG
 
-#undef USE_ASM_X86_64
-#undef USE_ECMULT_STATIC_PRECOMPUTATION
-#undef USE_EXTERNAL_ASM
-#undef USE_EXTERNAL_DEFAULT_CALLBACKS
-#undef USE_FORCE_WIDEMUL_INT64
-#undef USE_FORCE_WIDEMUL_INT128
-#undef ECMULT_WINDOW_SIZE
-
-#define USE_WIDEMUL_64 1
 #define ECMULT_WINDOW_SIZE 15
 
 #endif /* USE_BASIC_CONFIG */

src/gen_context.c

@@ -4,14 +4,6 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
-/* Autotools creates libsecp256k1-config.h, of which ECMULT_GEN_PREC_BITS is needed.
-   ifndef guard so downstream users can define their own if they do not use autotools. */
-#if !defined(ECMULT_GEN_PREC_BITS)
-#include "libsecp256k1-config.h"
-#endif
-#define USE_BASIC_CONFIG 1
-#include "basic-config.h"
-
 #include "include/secp256k1.h"
 #include "assumptions.h"
 #include "util.h"
@@ -20,6 +12,12 @@
 #include "group_impl.h"
 #include "ecmult_gen_impl.h"
 
+/* It is tempting to include basic-config.h for a basic config to make the above
+   modules work. But these modules currently do not need a config, except
+   ECMULT_GEN_PREC_BITS, which we except to be passed on the compiler command
+   line directly. We don't include basic-config.h because this file would
+   attempt to redefine ECMULT_GEN_PREC_BITS. */
+
 static void default_error_callback_fn(const char* str, void* data) {
     (void)data;
     fprintf(stderr, "[libsecp256k1] internal consistency check failed: %s\n", str);