[mlir] Don't emit unused labels

[marbre]

Summary

Stop the Cpp target from emitting unused labels. The previosly generated
code generated warning if -Wunused-label is passed to a compiler.

Co-authored-by: Simon Camphausen <[e-mail]>

[jpienaar]

mlir/lib/Target/Cpp/TranslateToCpp.cpp

634

Could you update this comment?

[marbre]

Updating D118154: [mlir] Don't emit unused labels

[marbre]

mlir/lib/Target/Cpp/TranslateToCpp.cpp

634

I've updated the comment and further revised the condition (it is sufficient to just check if the block has predecessors).

mlir/lib/Target/Cpp/TranslateToCpp.cpp

for (BlockArgument &arg : block.getArguments()) {
               emitter.emitType(block.getParentOp()->getLoc(), arg.getType()))) {
         return failure();
       }
       os << " " << emitter.getOrCreateName(arg) << ";\n";
     }
   }
 
   for (Block &block : blocks) {
-    // Only print a label if there is more than one block.
-    if (blocks.size() > 1) {
+    // Only print a label if the block has predecessors.
+    if (!block.hasNoPredecessors()) {
       if (failed(emitter.emitLabel(block)))
         return failure();
     }
     for (Operation &op : block.getOperations()) {
       // When generating code for an scf.if or std.cond_br op no semicolon needs
       // to be printed after the closing brace.
       // When generating code for an scf.for op, printing a trailing semicolon
       // is handled within the printOperation function.

mlir/test/Target/Cpp/control_flow.mlir

^bb4(%d : i64, %e : i64):
   return %0 : i64
 }
   // CPP-DECLTOP: int64_t simple(int64_t [[A:[^ ]*]], bool [[COND:[^ ]*]]) {
     // CPP-DECLTOP-NEXT: int64_t [[B:[^ ]*]];
     // CPP-DECLTOP-NEXT: int64_t [[V0:[^ ]*]];
     // CPP-DECLTOP-NEXT: int64_t [[C:[^ ]*]];
     // CPP-DECLTOP-NEXT: int64_t [[D:[^ ]*]];
     // CPP-DECLTOP-NEXT: int64_t [[E:[^ ]*]];
-    // CPP-DECLTOP-NEXT: [[BB0:[^ ]*]]:
     // CPP-DECLTOP-NEXT: if ([[COND]]) {
     // CPP-DECLTOP-NEXT: goto [[BB1:[^ ]*]];
     // CPP-DECLTOP-NEXT: } else {
     // CPP-DECLTOP-NEXT: goto [[BB2:[^ ]*]];
     // CPP-DECLTOP-NEXT: }
     // CPP-DECLTOP-NEXT: [[BB1]]:
     // CPP-DECLTOP-NEXT: [[C]] = [[A]];
     // CPP-DECLTOP-NEXT: goto [[BB3:[^ ]*]];

 
 func @block_labels0() {
 ^bb1:
     br ^bb2
 ^bb2:
     return
 }
 // CPP-DECLTOP: void block_labels0() {
-  // CPP-DECLTOP-NEXT: label1:
   // CPP-DECLTOP-NEXT: goto label2;
   // CPP-DECLTOP-NEXT: label2:
   // CPP-DECLTOP-NEXT: return;
   // CPP-DECLTOP-NEXT: }
 
 
 // Repeat the same function to make sure the names of the block labels get reset.
 func @block_labels1() {
 ^bb1:
     br ^bb2
 ^bb2:
     return
 }
 // CPP-DECLTOP: void block_labels1() {
-  // CPP-DECLTOP-NEXT: label1:
   // CPP-DECLTOP-NEXT: goto label2;
   // CPP-DECLTOP-NEXT: label2:
   // CPP-DECLTOP-NEXT: return;
   // CPP-DECLTOP-NEXT: }