Merge #18843: build: warn on potentially uninitialized reads

71f183a build: warn on potentially uninitialized reads (Vasil Dimov)

Pull request description:

  * Enable `conditional-uninitialized` warning class to show 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.

ACKs for top commit:
  practicalswift:
    ACK 71f183a
  laanwj:
    ACK 71f183a

Tree-SHA512: 2c1d8caacd86424b16a9d92e5df19e0bedb51ae111eecad7e3bfa46447bc88e5fff1f32dacf6c4a28257ebb3d87e79f80f074ce2c523ce08b1a0c0a67ab44204

Author: laanwj

configure.ac

@@ -344,6 +344,7 @@ if test "x$enable_werror" = "xyes"; then
   AX_CHECK_COMPILE_FLAG([-Werror=unused-variable],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=unused-variable"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=date-time],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=date-time"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Werror=return-type],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=return-type"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Werror=conditional-uninitialized],[ERROR_CXXFLAGS="$ERROR_CXXFLAGS -Werror=conditional-uninitialized"],,[[$CXXFLAG_WERROR]])
 fi
 
 if test "x$CXXFLAGS_overridden" = "xno"; then
@@ -359,6 +360,7 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   AX_CHECK_COMPILE_FLAG([-Wredundant-decls],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wredundant-decls"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wunused-variable],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wunused-variable"],,[[$CXXFLAG_WERROR]])
   AX_CHECK_COMPILE_FLAG([-Wdate-time],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wdate-time"],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-Wconditional-uninitialized],[WARN_CXXFLAGS="$WARN_CXXFLAGS -Wconditional-uninitialized"],,[[$CXXFLAG_WERROR]])
 
   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 +116,10 @@ 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;
+    // Initialize to 0 to silence a compiler warning that r1 or r2 may be used
+    // uninitialized. Even if rdrand fails (!ok) it will set the output to 0,
+    // but there is no way that the compiler could know that.
+    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 +131,7 @@ 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; // See above why we initialize to 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;