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tcg/README: Expand advice on number of TCG ops per target insn

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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.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
This commit is contained in:
Peter Maydell 2011-06-22 15:40:06 +01:00 committed by Blue Swirl
parent cf973e469b
commit 107a47cc2d
1 changed files with 9 additions and 1 deletions
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@ -504,7 +504,15 @@ register.
- Don't hesitate to use helpers for complicated or seldom used target - Don't hesitate to use helpers for complicated or seldom used target
instructions. There is little performance advantage in using TCG to instructions. There is little performance advantage in using TCG to
implement target instructions taking more than about twenty TCG 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 - 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 prove that a given global is "dead" at a given program point. The