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[WebAssembly][NFC] Document stackifier tablegen backend

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Summary:
Add comments to help readers avoid having to read tablegen backends to
understand the code. Also remove unecessary breaks from the output.

Reviewers: dschuff, aheejin

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D51371

llvm-svn: 340864
This commit is contained in:
Thomas Lively 2018-08-28 18:49:47 +00:00
parent c155591023
commit 64c3acad23
2 changed files with 19 additions and 3 deletions
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9
lib/Target/WebAssembly/WebAssemblyMCInstLower.cpp
View File

@ -285,8 +285,13 @@ static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI) {
static unsigned regInstructionToStackInstruction(unsigned OpCode) {
// For most opcodes, this function could have been implemented as "return
// OpCode + 1", but since table-gen alphabetically sorts them, this cannot be
// guaranteed (see e.g. BR and BR_IF), so we table-gen a giant switch
// statement instead.
// guaranteed (see e.g. BR and BR_IF). Instead we use a giant switch statement
// generated by a custom TableGen backend (WebAssemblyStackifierEmitter.cpp)
// that emits switch cases of the form
//
// case WebAssembly::RegisterInstr: return WebAssembly::StackInstr;
//
// for every pair of equivalent register and stack instructions.
switch (OpCode) {
default:
llvm_unreachable(

13
utils/TableGen/WebAssemblyStackifierEmitter.cpp
View File

@ -17,6 +17,17 @@
namespace llvm {
// Find all register WebAssembly instructions and their corresponding stack
// instructions. For each pair, emit a switch case of the form
//
// case WebAssembly::RegisterInstr: return WebAssembly::StackInstr;
//
// For example,
//
// case WebAssembly::ADD_I32: return WebAssembly::ADD_I32_S;
//
// This is useful for converting instructions from their register form to their
// equivalent stack form.
void EmitWebAssemblyStackifier(RecordKeeper &RK, raw_ostream &OS) {
Record *InstrClass = RK.getClass("WebAssemblyInst");
for (auto &RecordPair : RK.getDefs()) {
@ -26,7 +37,7 @@ void EmitWebAssemblyStackifier(RecordKeeper &RK, raw_ostream &OS) {
if (IsStackBased)
continue;
OS << " case WebAssembly::" << RecordPair.first << ": return "
<< "WebAssembly::" << RecordPair.first << "_S; break;\n";
<< "WebAssembly::" << RecordPair.first << "_S;\n";
}
}