LLL: rewrite alloc to avoid issues with edge cases.

1 files changed, 22 insertions, 6 deletions

diff --git a/liblll/CodeFragment.cpp b/liblll/CodeFragment.cpp
index 56c1e26a..f637dfb1 100644
--- a/liblll/CodeFragment.cpp
+++ b/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;
 		}