LLL: rewrite alloc to avoid issues with edge cases.

liblll/CodeFragment.cpp

@@ -523,14 +523,30 @@ void CodeFragment::constructOperation(sp::utree const& _t, CompilerState& _s)
 			requireSize(1);
 			requireDeposit(0, 1);
 
-			m_asm.append(Instruction::MSIZE);
-			m_asm.append(u256(0));
+			// (alloc N):
+			//  - Evaluates to (msize) before the allocation - the start of the allocated memory
+			//  - Does not allocate memory when N is zero
+			//  - Size of memory allocated is N bytes rounded up to a multiple of 32
+			//  - Uses MLOAD to expand MSIZE to avoid modifying memory.
+
+			auto end = m_asm.newTag();
+			m_asm.append(Instruction::MSIZE); // Result will be original top of memory
+			m_asm.append(code[0].m_asm, 1);	  // The alloc argument N
+			m_asm.append(Instruction::DUP1);
+			m_asm.append(Instruction::ISZERO);// (alloc 0) does not change MSIZE
+			m_asm.appendJumpI(end);
 			m_asm.append(u256(1));
-			m_asm.append(code[0].m_asm, 1);
-			m_asm.append(Instruction::MSIZE);
+			m_asm.append(Instruction::DUP2);  // Copy N
+			m_asm.append(Instruction::SUB);	  // N-1
+			m_asm.append(u256(0x1f));         // Bit mask
+			m_asm.append(Instruction::NOT);	  // Invert
+			m_asm.append(Instruction::AND);	  // Align N-1 on 32 byte boundary
+			m_asm.append(Instruction::MSIZE); // MSIZE is cheap
 			m_asm.append(Instruction::ADD);
-			m_asm.append(Instruction::SUB);
-			m_asm.append(Instruction::MSTORE8);
+			m_asm.append(Instruction::MLOAD); // Updates MSIZE
+			m_asm.append(Instruction::POP);	  // Discard the result of the MLOAD
+			m_asm.append(end);
+			m_asm.append(Instruction::POP);	  // Discard duplicate N
 
 			_s.usedAlloc = true;
 		}

liblll/CompilerState.cpp

@@ -46,6 +46,8 @@ void CompilerState::populateStandard()
 {
 	static const string s = "{"
 	"(def 'panic () (asm INVALID))"
+	// Alternative macro version of alloc, which is currently implemented in the parser
+	// "(def 'alloc (n) (raw (msize) (when n (pop (mload (+ (msize) (& (- n 1) (~ 0x1f))))))))"
 	"(def 'allgas (- (gas) 21))"
 	"(def 'send (to value) (call allgas to value 0 0 0 0))"
 	"(def 'send (gaslimit to value) (call gaslimit to value 0 0 0 0))"