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author | Raja R Harinath <harinath@hurrynot.org> | 2007-06-08 19:47:11 +0400 |
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committer | Raja R Harinath <harinath@hurrynot.org> | 2007-06-08 19:47:11 +0400 |
commit | 1b3ee868b15f4afd3fcc331e6fa430a6a75a87f0 (patch) | |
tree | 2a0a8456b016d5f1f0a17cc4e6cc7040d5354f11 | |
parent | 0c884036fe16cc895a9c11353cddf084c05b8e9c (diff) |
Add a note about "invariant meaning in a block" and how we handle it.
svn path=/trunk/mcs/; revision=78965
-rwxr-xr-x | mcs/docs/compiler.txt | 46 |
1 files changed, 44 insertions, 2 deletions
diff --git a/mcs/docs/compiler.txt b/mcs/docs/compiler.txt index 330d8a7977b..5a48476bbb0 100755 --- a/mcs/docs/compiler.txt +++ b/mcs/docs/compiler.txt @@ -476,6 +476,50 @@ ** Statements +*** Invariant meaning in a block + + The seemingly small section in the standard entitled + "invariant meaning in a block" has several subtleties + involved, especially when we try to implement the semantics + efficiently. + + Most of the semantics are trivial, and basically prevent local + variables from shadowing parameters and other local variables. + However, this notion is not limited to that, but affects all + simple name accesses within a block. And therein lies the rub + -- instead of just worrying about the issue when we arrive at + variable declarations, we need to verify this property at + every use of a simple name within a block. + + The key notion that helps us is to note the bi-directional + action of a variable declaration. The declaration together + with anti-shadowing rules can maintain the IMiaB property for + the block containing the declaration and all nested sub + blocks. But, the IMiaB property also forces all surrounding + blocks to avoid using the name. We thus need to maintain a + blacklist of taboo names in all surrounding blocks -- and we + take the expedient of doing so simply: actually maintaining a + (superset of the) blacklist in each block datastructure, which + we call the 'known_variable' list. + + Because we create the 'known_variable' list during the parse + process, by the time we do simple name resolution, all the + blacklists are fully populated. So, we can just enforce the + rest of the IMiaB property by looking up a couple of lists. + + This turns out to be quite efficient: when we used a block + tree walk, a testcase took 5-10mins, while with this simple + mildly-redundant data structure, the time taken for the same + testcase came down to a couple of seconds. + + The IKnownVariable interface is a small wrinkle. Firstly, the + IMiaB also applies to parameter names, especially those of + anonymous methods. Secondly, we need more information than + just the name in the blacklist -- we need the location of the + name and where it's declared. We use the IKnownVariable + interface to abstract out the parser information stored for + local variables and parameters. + * The semantic analysis Hence, the compiler driver has to parse all the input files. @@ -817,5 +861,3 @@ Once you have a full build of mcs, you can improve your development time by just issuing make in the `mcs' directory or using `make qh' in the gmcs directory. - -
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