V3MergeCond merges consecutive conditional `_ = cond ? _ : _` and
`if (cond) ...` statements. This patch adds an analysis and ordering
phase that moves statements with identical conditions closer to each
other, in order to enable more merging opportunities. This in turn
eliminates a lot of repeated conditionals which reduced dynamic branch
count and branch misprediction rate. Observed 6.5% improvement on
multi-threaded large designs, at the cost of less than 2% increase in
Verilation speed.
The _CONST suffix on these macros is only lexical notation, pointer
constness can be preserved by overloading the underlying
implementations appropriately. Given that the compiler will catch
invalid const usage (trying to assign a non-const pointer to a const
pointer variable, etc.), and that the declarations of symbols should
make their constness obvious, I see no reason to keep the _CONST
flavours.
- Merge AstNodeIf nodes as well (not just assignment from AstCond)
- Merge merged results recursively (optimizes nested conditionals/ifs)
- Only checking mergeability once per node.
- Don't add redundant masking
- Duplicate cheap statements in both branches, if doing so yields a
larger merge
- Include reduced nodes before the starting conditional in the merge
- Do try to merge after assignment to condition when possible.
- Do not try to merge reduced form if not the expected statement.
This used to cause a crash.
This provides minor simulation performance benefit, but can provide
large C++ compilation time improvement, notably with Clang (4x).
This patch implements #2366 .
