diff options
Diffstat (limited to 'thesis')
| -rw-r--r-- | thesis/storing-pc.tex | 43 |
1 files changed, 41 insertions, 2 deletions
diff --git a/thesis/storing-pc.tex b/thesis/storing-pc.tex index 4bae7fb..d3fc474 100644 --- a/thesis/storing-pc.tex +++ b/thesis/storing-pc.tex @@ -173,14 +173,53 @@ A general comparison of running time under ARM and Thumb is made in \cref{sec:re \subsection{Other solutions} \label{sec:storing-pc:other-solutions} -Another solution than the one we present makes use of the link register. +Another solution than the one we have considered here makes use of the link register. Some branch instructions, like \ual{bl}, store the address of the next instruction in the link register. We could therefore imagine a setup where the callee gets the return address from that register rather than from the stack. This is the approach taken by GCC. The code of a typical C subroutine starts with \ual{push {...,lr}} and ends with \ual{pop {...,pc}}. When generating code for a functional language, it is not straightforward to do this, due to tail recursion. -It is an easier solution to have the caller responsible for storing the return address, +An example of this can be found in the following basic function: + +\begin{minted}{clean} + length :: !Int ![a] -> Int + length n [] = n + length n [_:xs] = length (n+1) xs +\end{minted} + +The current Thumb backend generates the following code: + +\begin{minted}{ual} +slength_P2: + ldr r0,[r6] @ Load top of A-stack (the list) + ldr r14,=__Nil+2 @ Check if it is Nil + cmp r0,r14 + bne case_P2 +case_P1: @ If the list is Nil + ldr pc,[sp],#4 @ Simply return + .ltorg +case_P2: @ If the list is not Nil + ldr r6,[r6,#8] @ Load the Cons part of the list + ldr r0,[r6] @ Check if it has been evaluated already + tst r0,#2 + bne e_0 + str r4,[sp,#-4]! @ If the Cons part has not been evaluated + .align + add.w r14,pc,#7 @ Evaluate the cons part + str.w r14,[sp,#-4]! + blx r0 + ldr r4,[sp],#4 +e_0: @ If / after the Cons part has been evaluated + add r4,r4,#1 @ Increase counter + b slength_P2 @ Tail recursively jump back to start of function + .ltorg +\end{minted} + +In this setup, it is not easily possible to let the callee store the return address, + since we enter the function as many times as there are elements in the list, + while we only return from it once, in \texttt{case\_P1}. +A more straightforward solution to have the caller responsible for storing the return address, which is why this approach is taken in Clean's ARM code generator~\parencite{armcg} and why we continue along these lines for the Thumb backend. |