Stopgap for rewrite-being-too-large

Just let the rewrite continue but stop emitting any assembly,
and then at the end abort the rewrite.

src/asm_writing/assembler.cpp

@@ -98,6 +98,11 @@ void Assembler::emitArith(Immediate imm, Register r, int opcode) {
 
 
 void Assembler::emitByte(uint8_t b) {
+    if (addr >= end_addr) {
+        failed = true;
+        return;
+    }
+
     assert(addr < end_addr);
     *addr = b;
     ++addr;

src/asm_writing/assembler.h

@@ -51,6 +51,7 @@ class Assembler {
 private:
     uint8_t* const start_addr, *const end_addr;
     uint8_t* addr;
+    bool failed; // if the rewrite failed at the assembly-generation level for some reason
 
     static const uint8_t OPCODE_ADD = 0b000, OPCODE_SUB = 0b101;
     static const uint8_t REX_B = 1, REX_X = 2, REX_R = 4, REX_W = 8;
@@ -64,7 +65,9 @@ private:
     void emitArith(Immediate imm, Register reg, int opcode);
 
 public:
-    Assembler(uint8_t* start, int size) : start_addr(start), end_addr(start + size), addr(start_addr) {}
+    Assembler(uint8_t* start, int size) : start_addr(start), end_addr(start + size), addr(start_addr), failed(false) {}
+
+    bool hasFailed() { return failed; }
 
     void nop() { emitByte(0x90); }
     void trap() { emitByte(0xcc); }

src/asm_writing/rewriter.cpp

@@ -664,6 +664,15 @@ void Rewriter::abort() {
 
 void Rewriter::commit() {
     assert(!finished);
+
+    if (assembler->hasFailed()) {
+        static StatCounter rewriter_assemblyfail("rewriter_assemblyfail");
+        rewriter_assemblyfail.log();
+
+        this->abort();
+        return;
+    }
+
     finished = true;
 
     static StatCounter rewriter_commits("rewriter_commits");