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Diffstat (limited to 'thesis')
| -rw-r--r-- | thesis/storing-pc.tex | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/thesis/storing-pc.tex b/thesis/storing-pc.tex index 81a688b..40da1d1 100644 --- a/thesis/storing-pc.tex +++ b/thesis/storing-pc.tex @@ -128,6 +128,27 @@ 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{Implementation details} +\label{sec:storing-pc:implementation} +A jump always jumps to either a label (with \ual{b} or \ual{bl}) or a register (with \ual{bx} or \ual{blx}). +The latter occurs for example in the case of a \abc{jsr_eval} ABC instruction. +This instruction is used to evaluate a node on the A-stack. +First, the node entry address has to be fetched; then we jump to that address. + +In the first case, we need one scratch register to store the PC temporarily. +In the second case, we need two scratch registers: also one for the address we are jumping to. +For the ARM instruction set we needed zero and one scratch register(s), respectively. +The ARM backend uses two scratch registers, S0 and S1 (for more details, see \cref{sec:reg-alloc:clean}). +The latter is used only when two scratch registers are needed, so S0 is used in this case. + +The Thumb-2 code generator uses S0 to store the PC temporarily in the first case, + and S1 in the second case (where S0 is still used for the address we are jumping to). +This makes sure that S0 is used as much as possible. +The slightly easier implementation would use S1 in both cases. +However, in Thumb-2 it is convenient to have great variation in register usage: + this allows for a massive code size optimisation (see \cref{sec:reg-alloc}). +For this reason it is better to use S0 whenever possible. + \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. |