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Diffstat (limited to 'thesis/storing-pc.tex')
| -rw-r--r-- | thesis/storing-pc.tex | 27 |
1 files changed, 14 insertions, 13 deletions
diff --git a/thesis/storing-pc.tex b/thesis/storing-pc.tex index e08fb4d..81a688b 100644 --- a/thesis/storing-pc.tex +++ b/thesis/storing-pc.tex @@ -128,20 +128,8 @@ The offset, 9, is calculated as the number of bytes to the instruction after the For \ual{b} and \ual{bl} instructions, this means an offset of 9, since these instructions are 32-bit. The \ual{bx} and \ual{blx} instructions are 16-bit, and require an offset of 7. -\subsection{Other solutions} -\label{sec:storing-pc:other-solutions} -Another solution than the one we present 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, - 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. - \subsection{Comparison} +\label{sec:storing-pc:comparison} Assuming the worst case, that all instructions in the jump block are wide, we need four more bytes in Thumb than in ARM. As a benchmark, the Clean compiler has 41,006 jumps of this kind in 1,253,978 instructions, a rough 3.27\%. The four extra bytes in Thumb mean a size increase of $41006\cdot4\approx160$KiB on the 5.3MiB file, an increase of 3.00\%. @@ -157,4 +145,17 @@ A general comparison of running time under ARM and Thumb is made in \cref{sec:re % pi@rasppi:~/clean/exe$ objdump -d cocl | grep -E '^\s+[0123456789abcdef]{5,8}:\s+[0123456789abcdef]{8}' | wc -l % 1253978 +\subsection{Other solutions} +\label{sec:storing-pc:other-solutions} +Another solution than the one we present 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, + 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. + \end{multicols} |