[all-commits] [llvm/llvm-project] 1d9291: [MC] Rewrite tablegen for printInstrAlias to comip...

[Reid Kleckner via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 1d9291cc785c453ac189d131271e91f8aaf6858c
      https://github.com/llvm/llvm-project/commit/1d9291cc785c453ac189d131271e91f8aaf6858c
  Author: Reid Kleckner <rnk at google.com>
  Date:   2019-12-06 (Fri, 06 Dec 2019)

  Changed paths:
    M llvm/include/llvm/MC/MCInstPrinter.h
    M llvm/lib/MC/MCInstPrinter.cpp
    M llvm/utils/TableGen/AsmWriterEmitter.cpp

  Log Message:
  -----------
  [MC] Rewrite tablegen for printInstrAlias to comiple faster, NFC

Before this change, the *InstPrinter.cpp files of each target where some
of the slowest objects to compile in all of LLVM. See this snippet produced by
ClangBuildAnalyzer:
https://reviews.llvm.org/P8171$96
Search for "InstPrinter", and see that it shows up in a few places.

Tablegen was emitting a large switch containing a sequence of operand checks,
each of which created many conditions and many BBs. Register allocation and
jump threading both did not scale well with such a large repetitive sequence of
basic blocks.

So, this change essentially turns those control flow structures into
data. The previous structure looked like:

  switch (Opc) {
  case TGT::ADD:
    // check alias 1
    if (MI->getOperandCount() == N && // check num opnds
        MI->getOperand(0).isReg() && // check opnd 0
        ...
        MI->getOperand(1).isImm() && // check opnd 1
     AsmString = "foo";
     break;
   }
   // check alias 2
   if (...)
     ...
   return false;

The new structure looks like:

  OpToPatterns: Sorted table of opcodes mapping to pattern indices.
   \->
     Patterns: List of patterns. Previous table points to subrange of
               patterns to match.
      \->
        Conds: The if conditions above encoded as a kind and 32-bit value.

See MCInstPrinter.cpp for the details of how the new data structures are
interpreted.

Here are some before and after metrics.
Time to compile AArch64InstPrinter.cpp:
  0m29.062s vs. 0m2.203s
size of the obj:
  3.9M vs. 676K
size of clang.exe:
  97M vs. 96M

I have not benchmarked disassembly performance, but typically
disassemblers are bottlenecked on IO and string processing, not alias
matching, so I'm not sure it's interesting enough to be worth doing.

Reviewers: RKSimon, andreadb, xbolva00, craig.topper

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D70650