Address some `-Wsign-compare` warnings.

Fixes #499.

Change-Id: I076d4fb4a51e0589176fdd175045ac90b0e361cb
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63350
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

BUILD.bazel

diff --git a/BUILD.bazel b/BUILD.bazel
index c765cfd..643d71f 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -135,6 +135,7 @@
         ":re2",
         "@abseil-cpp//absl/base",
         "@abseil-cpp//absl/base:core_headers",
+        "@abseil-cpp//absl/container:flat_hash_set",
         "@abseil-cpp//absl/flags:flag",
         "@abseil-cpp//absl/log:absl_check",
         "@abseil-cpp//absl/log:absl_log",

re2/bitmap256.h

diff --git a/re2/bitmap256.h b/re2/bitmap256.h
index e16a570..e303e71 100644
--- a/re2/bitmap256.h
+++ b/re2/bitmap256.h
@@ -51,7 +51,7 @@
  private:
   // Finds the least significant non-zero bit in n.
   static int FindLSBSet(uint64_t n) {
-    ABSL_DCHECK_NE(n, 0);
+    ABSL_DCHECK_NE(n, uint64_t{0});
 #if defined(__GNUC__)
     return __builtin_ctzll(n);
 #elif defined(_MSC_VER) && defined(_M_X64)

re2/prog.cc

diff --git a/re2/prog.cc b/re2/prog.cc
index d3ff26e..9f1cc00 100644
--- a/re2/prog.cc
+++ b/re2/prog.cc
@@ -37,13 +37,13 @@
 // Constructors per Inst opcode
 
 void Prog::Inst::InitAlt(uint32_t out, uint32_t out1) {
-  ABSL_DCHECK_EQ(out_opcode_, 0);
+  ABSL_DCHECK_EQ(out_opcode_, uint32_t{0});
   set_out_opcode(out, kInstAlt);
   out1_ = out1;
 }
 
 void Prog::Inst::InitByteRange(int lo, int hi, int foldcase, uint32_t out) {
-  ABSL_DCHECK_EQ(out_opcode_, 0);
+  ABSL_DCHECK_EQ(out_opcode_, uint32_t{0});
   set_out_opcode(out, kInstByteRange);
   lo_ = lo & 0xFF;
   hi_ = hi & 0xFF;
@@ -51,30 +51,30 @@
 }
 
 void Prog::Inst::InitCapture(int cap, uint32_t out) {
-  ABSL_DCHECK_EQ(out_opcode_, 0);
+  ABSL_DCHECK_EQ(out_opcode_, uint32_t{0});
   set_out_opcode(out, kInstCapture);
   cap_ = cap;
 }
 
 void Prog::Inst::InitEmptyWidth(EmptyOp empty, uint32_t out) {
-  ABSL_DCHECK_EQ(out_opcode_, 0);
+  ABSL_DCHECK_EQ(out_opcode_, uint32_t{0});
   set_out_opcode(out, kInstEmptyWidth);
   empty_ = empty;
 }
 
 void Prog::Inst::InitMatch(int32_t id) {
-  ABSL_DCHECK_EQ(out_opcode_, 0);
+  ABSL_DCHECK_EQ(out_opcode_, uint32_t{0});
   set_opcode(kInstMatch);
   match_id_ = id;
 }
 
 void Prog::Inst::InitNop(uint32_t out) {
-  ABSL_DCHECK_EQ(out_opcode_, 0);
+  ABSL_DCHECK_EQ(out_opcode_, uint32_t{0});
   set_opcode(kInstNop);
 }
 
 void Prog::Inst::InitFail() {
-  ABSL_DCHECK_EQ(out_opcode_, 0);
+  ABSL_DCHECK_EQ(out_opcode_, uint32_t{0});
   set_opcode(kInstFail);
 }
 
@@ -1113,7 +1113,7 @@
 #if defined(__AVX2__)
 // Finds the least significant non-zero bit in n.
 static int FindLSBSet(uint32_t n) {
-  ABSL_DCHECK_NE(n, 0);
+  ABSL_DCHECK_NE(n, uint32_t{0});
 #if defined(__GNUC__)
   return __builtin_ctz(n);
 #elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_IX86))
@@ -1135,7 +1135,7 @@
 #endif
 
 const void* Prog::PrefixAccel_FrontAndBack(const void* data, size_t size) {
-  ABSL_DCHECK_GE(prefix_size_, 2);
+  ABSL_DCHECK_GE(prefix_size_, size_t{2});
   if (size < prefix_size_)
     return NULL;
   // Don't bother searching the last prefix_size_-1 bytes for prefix_front_.

re2/re2.cc

diff --git a/re2/re2.cc b/re2/re2.cc
index 2e25b64..5978035 100644
--- a/re2/re2.cc
+++ b/re2/re2.cc
@@ -332,7 +332,7 @@
 
 // Finds the most significant non-zero bit in n.
 static int FindMSBSet(uint32_t n) {
-  ABSL_DCHECK_NE(n, 0);
+  ABSL_DCHECK_NE(n, uint32_t{0});
 #if defined(__GNUC__)
   return 31 ^ __builtin_clz(n);
 #elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_IX86))

re2/regexp.cc

diff --git a/re2/regexp.cc b/re2/regexp.cc
index 1e5ae90..f7e5ba2 100644
--- a/re2/regexp.cc
+++ b/re2/regexp.cc
@@ -498,7 +498,7 @@
     if (n == 0)
       break;
 
-    ABSL_DCHECK_GE(n, 2);
+    ABSL_DCHECK_GE(n, size_t{2});
     a = stk[n-2];
     b = stk[n-1];
     stk.resize(n-2);