compiler: create temporary allocas with appropriate lifetimes

Make sure all allocas are created in the entry block and are given the
right lifetimes. This is good for code quality:

  * Moving allocas to the entry block makes sure they are always
    allocated statically (avoiding the need for a frame pointer) and do
    not grow the stack on each new alloca instruction. This is
    especially useful in loops where it could otherwise lead to a stack
    overflow even though there is no recursion.
  * Adding lifetime markers allows LLVM to reuse stack areas for
    different allocas as long as their lifetimes do not overlap.

All in all, this reduces code size in all tested cases for the BBC
micro:bit, and reduces code size for most cases for WebAssembly.

compiler/wordpack.go

@@ -34,7 +34,7 @@ func (c *Compiler) emitPointerPack(values []llvm.Value) llvm.Value {
 		}
 		// Because packedType is a struct and we have to cast it to a *i8, store
 		// it in an alloca first for bitcasting (store+bitcast+load).
-		packedAlloc = c.builder.CreateAlloca(packedType, "")
+		packedAlloc, _, _ = c.createTemporaryAlloca(packedType, "")
 	} else {
 		// Packed data is bigger than a pointer, so allocate it on the heap.
 		sizeValue := llvm.ConstInt(c.uintptrType, size, false)
@@ -54,7 +54,11 @@ func (c *Compiler) emitPointerPack(values []llvm.Value) llvm.Value {
 	if packedHeapAlloc.IsNil() {
 		// Load value (as *i8) from the alloca.
 		packedAlloc = c.builder.CreateBitCast(packedAlloc, llvm.PointerType(c.i8ptrType, 0), "")
-		return c.builder.CreateLoad(packedAlloc, "")
+		result := c.builder.CreateLoad(packedAlloc, "")
+		packedPtr := c.builder.CreateBitCast(packedAlloc, c.i8ptrType, "")
+		packedSize := llvm.ConstInt(c.ctx.Int64Type(), c.targetData.TypeAllocSize(packedAlloc.Type()), false)
+		c.emitLifetimeEnd(packedPtr, packedSize)
+		return result
 	} else {
 		// Get the original heap allocation pointer, which already is an *i8.
 		return packedHeapAlloc
@@ -66,7 +70,7 @@ func (c *Compiler) emitPointerUnpack(ptr llvm.Value, valueTypes []llvm.Type) []l
 	packedType := c.ctx.StructType(valueTypes, false)
 
 	// Get a correctly-typed pointer to the packed data.
-	var packedAlloc llvm.Value
+	var packedAlloc, packedRawAlloc llvm.Value
 	size := c.targetData.TypeAllocSize(packedType)
 	if size == 0 {
 		// No data to unpack.
@@ -80,7 +84,7 @@ func (c *Compiler) emitPointerUnpack(ptr llvm.Value, valueTypes []llvm.Type) []l
 			return []llvm.Value{c.builder.CreatePtrToInt(ptr, valueTypes[0], "unpack.int")}
 		}
 		// Fallback: load it using an alloca.
-		packedRawAlloc := c.builder.CreateAlloca(llvm.PointerType(c.i8ptrType, 0), "unpack.raw.alloc")
+		packedRawAlloc, _, _ = c.createTemporaryAlloca(llvm.PointerType(c.i8ptrType, 0), "unpack.raw.alloc")
 		packedRawValue := c.builder.CreateBitCast(ptr, llvm.PointerType(c.i8ptrType, 0), "unpack.raw.value")
 		c.builder.CreateStore(packedRawValue, packedRawAlloc)
 		packedAlloc = c.builder.CreateBitCast(packedRawAlloc, llvm.PointerType(packedType, 0), "unpack.alloc")
@@ -104,5 +108,10 @@ func (c *Compiler) emitPointerUnpack(ptr llvm.Value, valueTypes []llvm.Type) []l
 		gep := c.builder.CreateInBoundsGEP(packedAlloc, indices, "")
 		values[i] = c.builder.CreateLoad(gep, "")
 	}
+	if !packedRawAlloc.IsNil() {
+		allocPtr := c.builder.CreateBitCast(packedRawAlloc, c.i8ptrType, "")
+		allocSize := llvm.ConstInt(c.ctx.Int64Type(), c.targetData.TypeAllocSize(c.uintptrType), false)
+		c.emitLifetimeEnd(allocPtr, allocSize)
+	}
 	return values
 }