build: warn on potentially uninitialized reads

Enable -Wconditional-uninitialized to warn on potentially uninitialized
reads.

Fix the sole such warning in Bitcoin Core in GetRdRand(): r1 would be
set to 0 on rdrand failure, so initializing it to 0 is a non-functional
change.

Author: vasild

configure.ac

@@ -349,6 +349,7 @@ if test "x$enable_werror" = "xyes"; then
   APPEND_FLAG_IF_AVAILABLE([ERROR_CXXFLAGS], [-Werror=unused-variable])
   APPEND_FLAG_IF_AVAILABLE([ERROR_CXXFLAGS], [-Werror=date-time])
   APPEND_FLAG_IF_AVAILABLE([ERROR_CXXFLAGS], [-Werror=return-type])
+  APPEND_FLAG_IF_AVAILABLE([ERROR_CXXFLAGS], [-Werror=conditional-uninitialized])
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then
@@ -363,6 +364,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   APPEND_FLAG_IF_AVAILABLE([WARN_CXXFLAGS], [-Wredundant-decls])
   APPEND_FLAG_IF_AVAILABLE([WARN_CXXFLAGS], [-Wunused-variable])
   APPEND_FLAG_IF_AVAILABLE([WARN_CXXFLAGS], [-Wdate-time])
+  APPEND_FLAG_IF_AVAILABLE([WARN_CXXFLAGS], [-Wconditional-uninitialized])
 
   dnl Some compilers (gcc) ignore unknown -Wno-* options, but warn about all
   dnl unknown options if any other warning is produced. Test the -Wfoo case, and

src/Makefile.leveldb.include

@@ -36,7 +36,7 @@ LEVELDB_CPPFLAGS_INT += -DLEVELDB_PLATFORM_POSIX
 endif
 
 leveldb_libleveldb_a_CPPFLAGS = $(AM_CPPFLAGS) $(LEVELDB_CPPFLAGS_INT) $(LEVELDB_CPPFLAGS)
-leveldb_libleveldb_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
+leveldb_libleveldb_a_CXXFLAGS = $(filter-out -Wconditional-uninitialized -Werror=conditional-uninitialized, $(AM_CXXFLAGS)) $(PIE_FLAGS)
 
 leveldb_libleveldb_a_SOURCES=
 leveldb_libleveldb_a_SOURCES += leveldb/port/port_stdcxx.h

src/random.cpp

@@ -116,7 +116,7 @@ static uint64_t GetRdRand() noexcept
     // RdRand may very rarely fail. Invoke it up to 10 times in a loop to reduce this risk.
 #ifdef __i386__
     uint8_t ok;
-    uint32_t r1, r2;
+    uint32_t r1 = 0, r2 = 0;
     for (int i = 0; i < 10; ++i) {
         __asm__ volatile (".byte 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %eax
         if (ok) break;
@@ -128,7 +128,7 @@ static uint64_t GetRdRand() noexcept
     return (((uint64_t)r2) << 32) | r1;
 #elif defined(__x86_64__) || defined(__amd64__)
     uint8_t ok;
-    uint64_t r1;
+    uint64_t r1 = 0;
     for (int i = 0; i < 10; ++i) {
         __asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %rax
         if (ok) break;