Expand the note on the number of TCG ops generated per target insn, to be clearer about the range of applicability of the 20 op rule of thumb. Also add a note about the hard MAX_OP_PER_INSTR limit.
Signed-off-by: Peter Maydell <peter.mayd...@linaro.org> --- The expansion of the first bullet point is based on remarks by Aurelien in this email: http://lists.gnu.org/archive/html/qemu-devel/2011-06/msg00395.html tcg/README | 10 +++++++++- 1 files changed, 9 insertions(+), 1 deletions(-) diff --git a/tcg/README b/tcg/README index 6600122..cfdfd96 100644 --- a/tcg/README +++ b/tcg/README @@ -504,7 +504,15 @@ register. - Don't hesitate to use helpers for complicated or seldom used target instructions. There is little performance advantage in using TCG to implement target instructions taking more than about twenty TCG - instructions. + instructions. Note that this rule of thumb is more applicable to + helpers doing complex logic or arithmetic, where the C compiler has + scope to do a good job of optimisation; it is less relevant where + the instruction is mostly doing loads and stores, and in those cases + inline TCG may still be faster for longer sequences. + +- The hard limit on the number of TCG instructions you can generate + per target instruction is set by MAX_OP_PER_INSTR in exec-all.h -- + you cannot exceed this without risking a buffer overrun. - Use the 'discard' instruction if you know that TCG won't be able to prove that a given global is "dead" at a given program point. The -- 1.7.1
