Currently the 'MetricExpr' json value is passed from the json
file to the pmu-events.c. This change introduces an expression
tree that is parsed into. The parsing is done largely by using
operator overloading and python's 'eval' function. Two advantages
in doing this are:
1) Broken metrics fail at compile time rather than relying on
`perf test` to detect. `perf test` remains relevant for checking
event encoding and actual metric use.
2) The conversion to a string from the tree can minimize the metric's
string size, for example, preferring 1e6 over 1000000, avoiding
multiplication by 1 and removing unnecessary whitespace. On x86
this reduces the string size by 3,050bytes (0.07%).
In future changes it would be possible to programmatically
generate the json expressions (a single line of text and so a
pain to write manually) for an architecture using the expression
tree. This could avoid copy-pasting metrics for all architecture
variants.
Signed-off-by: Ian Rogers<irogers@xxxxxxxxxx>