Simplify PointerCompare a little

Technically makes a few behavioral changes, but these shouldn't have
any real consequences:

1. When comparing conditional branch statements, the 'false' jump
target is now compared lexicographically before the 'true' jump target
(to reduce redundancy with comparing unconditional branches).  This
affects block ordering slightly, but equality (which is what we really
care about) remains the same.

2. Adds a bit more sanity checking: RET and JMP instructions should
only occur at the end of basic blocks.

Review URL: https://codereview.chromium.org/286063007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271593 0039d316-1c4b-4281-b951-d872f2087c98

sandbox/linux/seccomp-bpf/codegen.cc

@@ -443,80 +443,74 @@ static int PointerCompare(const BasicBlock* block1,
     // If we have reached the end of the sequence of instructions in one or
     // both basic blocks, we know the relative ordering between the two blocks
     // and can return.
-    if (iter1 == insns1.end()) {
-      if (iter2 == insns2.end()) {
-        // If the two blocks are the same length (and have elementwise-equal
-        // code and k fields, which is the only way we can reach this point),
-        // and the last instruction isn't a JMP or a RET, then we must compare
-        // their successors.
-        Instruction* const insns1_last = insns1.back();
-        Instruction* const insns2_last = insns2.back();
-        if (BPF_CLASS(insns1_last->code) != BPF_JMP &&
-            BPF_CLASS(insns1_last->code) != BPF_RET) {
-          // Non jumping instructions will always have a valid next instruction.
-          CHECK(insns1_last->next);
-          CHECK(insns2_last->next);
-          return PointerCompare(blocks.find(insns1_last->next)->second,
-                                blocks.find(insns2_last->next)->second,
-                                blocks);
-        } else {
-          return 0;
-        }
+    if (iter1 == insns1.end() || iter2 == insns2.end()) {
+      if (iter1 != insns1.end()) {
+        return 1;
+      }
+      if (iter2 != insns2.end()) {
+        return -1;
       }
-      return -1;
-    } else if (iter2 == insns2.end()) {
-      return 1;
+
+      // If the two blocks are the same length (and have elementwise-equal code
+      // and k fields) and their last instructions are neither a JMP nor a RET
+      // (which is the only way we can reach this point), then we must compare
+      // their successors.
+      Instruction* const insns1_last = insns1.back();
+      Instruction* const insns2_last = insns2.back();
+      CHECK(BPF_CLASS(insns1_last->code) != BPF_JMP &&
+            BPF_CLASS(insns1_last->code) != BPF_RET);
+
+      // Non jumping instructions will always have a valid next instruction.
+      CHECK(insns1_last->next);
+      CHECK(insns2_last->next);
+      return PointerCompare(blocks.find(insns1_last->next)->second,
+                            blocks.find(insns2_last->next)->second,
+                            blocks);
     }
 
     // Compare the individual fields for both instructions.
     const Instruction& insn1 = **iter1;
     const Instruction& insn2 = **iter2;
-    if (insn1.code == insn2.code) {
-      if (insn1.k == insn2.k) {
-        // Only conditional jump instructions use the jt_ptr and jf_ptr
-        // fields.
-        if (BPF_CLASS(insn1.code) == BPF_JMP) {
-          if (BPF_OP(insn1.code) != BPF_JA) {
-            // Recursively compare the "true" and "false" branches.
-            // A well-formed BPF program can't have any cycles, so we know
-            // that our recursive algorithm will ultimately terminate.
-            // In the unlikely event that the programmer made a mistake and
-            // went out of the way to give us a cyclic program, we will crash
-            // with a stack overflow. We are OK with that.
-            int c = PointerCompare(blocks.find(insn1.jt_ptr)->second,
-                                   blocks.find(insn2.jt_ptr)->second,
-                                   blocks);
-            if (c == 0) {
-              c = PointerCompare(blocks.find(insn1.jf_ptr)->second,
-                                 blocks.find(insn2.jf_ptr)->second,
-                                 blocks);
-              if (c == 0) {
-                continue;
-              } else {
-                return c;
-              }
-            } else {
-              return c;
-            }
-          } else {
-            int c = PointerCompare(blocks.find(insn1.jt_ptr)->second,
-                                   blocks.find(insn2.jt_ptr)->second,
-                                   blocks);
-            if (c == 0) {
-              continue;
-            } else {
-              return c;
-            }
-          }
-        } else {
-          continue;
-        }
-      } else {
-        return insn1.k - insn2.k;
-      }
-    } else {
+    if (insn1.code != insn2.code) {
       return insn1.code - insn2.code;
     }
+    if (insn1.k != insn2.k) {
+      return insn1.k - insn2.k;
+    }
+
+    // Sanity check: If we're looking at a JMP or RET instruction, by definition
+    // it should be the last instruction of the basic block.
+    if (BPF_CLASS(insn1.code) == BPF_JMP || BPF_CLASS(insn1.code) == BPF_RET) {
+      CHECK_EQ(insns1.back(), &insn1);
+      CHECK_EQ(insns2.back(), &insn2);
+    }
+
+    // RET instructions terminate execution, and only JMP instructions use the
+    // jt_ptr and jf_ptr fields.  Anything else can continue to the next
+    // instruction in the basic block.
+    if (BPF_CLASS(insn1.code) == BPF_RET) {
+      return 0;
+    } else if (BPF_CLASS(insn1.code) != BPF_JMP) {
+      continue;
+    }
+
+    // Recursively compare the "true" and "false" branches.
+    // A well-formed BPF program can't have any cycles, so we know
+    // that our recursive algorithm will ultimately terminate.
+    // In the unlikely event that the programmer made a mistake and
+    // went out of the way to give us a cyclic program, we will crash
+    // with a stack overflow. We are OK with that.
+    if (BPF_OP(insn1.code) != BPF_JA) {
+      int c = PointerCompare(blocks.find(insn1.jf_ptr)->second,
+                             blocks.find(insn2.jf_ptr)->second,
+                             blocks);
+      if (c != 0) {
+        return c;
+      }
+    }
+    return PointerCompare(blocks.find(insn1.jt_ptr)->second,
+                          blocks.find(insn2.jt_ptr)->second,
+                          blocks);
   }
 }