[CIR] Fix error in cir.label when using cir.br in entry block (#1939)

This PR fixes an error I found while working on `cir.indirectbr`. The
issue occurs when a branching operator points to the entry block LLVM’s
verifier does not allow this
https://github.com/llvm/clangir/blob/10f2ee11fa61bb1550819ed54a5b0e111d9243aa/mlir/lib/IR/Verifier.cpp#L205-L208
Previously, in `cir.label`, when building a block, we only checked if
the current block was not empty. Now, we also check if we are in the
entry block. If we are, a new block is created instead.
This change also helps emit IR that is closer to the classic code
behavior.

Author: Andres-Salamanca

clang/lib/CIR/CodeGen/CIRGenStmt.cpp

@@ -643,7 +643,7 @@ mlir::LogicalResult CIRGenFunction::emitLabel(const LabelDecl *D) {
   // to this label.
   mlir::Block *currBlock = builder.getBlock();
   mlir::Block *labelBlock = currBlock;
-  if (!currBlock->empty()) {
+  if (!currBlock->empty() || currBlock->isEntryBlock()) {
     {
       mlir::OpBuilder::InsertionGuard guard(builder);
       labelBlock = builder.createBlock(builder.getBlock()->getParent());

clang/test/CIR/CodeGen/goto.cpp

@@ -68,6 +68,15 @@ int g2() {
 // CHECK-NEXT:     [[R]] : !s32i
 // CHECK-NEXT:   }
 
+void g3() {
+label:
+  goto label;
+}
+
+// CHECK:  cir.func dso_local @_Z2g3v
+// CHECK:    cir.br ^bb1
+// CHECK:  ^bb1:
+// CHECK:    cir.br ^bb1
 
 int shouldNotGenBranchRet(int x) {
   if (x > 5)
@@ -189,7 +198,7 @@ int jumpFromLoop(int* ar) {
   if (!ar) {
 err:
     return -1;
-}
+  }
 
   while (ar) {
     if (*ar == 42)
@@ -200,14 +209,16 @@ int jumpFromLoop(int* ar) {
   return 0;
 }
 // CHECK:  cir.func dso_local @_Z12jumpFromLoopPi
-// CHECK:    cir.brcond {{.*}} ^bb[[#RETURN1:]], ^bb[[#BLK3:]]
+// CHECK:    cir.brcond {{.*}} ^bb[[#LABELERR:]], ^bb[[#BLK4:]]
+// CHECK:  ^bb[[#LABELERR]]:
+// CHECK:    cir.br ^bb[[#RETURN1:]]
 // CHECK:  ^bb[[#RETURN1]]:
 // CHECK:    cir.return
-// CHECK:  ^bb[[#BLK3]]:
-// CHECK:    cir.br ^bb[[#BLK4:]]
 // CHECK:  ^bb[[#BLK4]]:
 // CHECK:    cir.br ^bb[[#BLK5:]]
 // CHECK:  ^bb[[#BLK5]]:
+// CHECK:    cir.br ^bb[[#BLK6:]]
+// CHECK:  ^bb[[#BLK6]]:
 // CHECK:    cir.br ^bb[[#COND:]]
 // CHECK:  ^bb[[#COND]]:
 // CHECK:    cir.brcond {{.*}} ^bb[[#BODY:]], ^bb[[#EXIT:]]
@@ -254,23 +265,27 @@ void flatLoopWithNoTerminatorInFront(int* ptr) {
 // CHECK:    cir.br ^bb[[#BLK5:]]
 // CHECK:  ^bb[[#BLK5]]:
 // CHECK:    cir.br ^bb[[#BODY:]]
-// CHECK:  ^bb[[#COND]]:
+// CHECK:  ^bb[[#COND:]]:
 // CHECK:    cir.brcond {{.*}} ^bb[[#BODY]], ^bb[[#EXIT:]]
 // CHECK:  ^bb[[#BODY]]:
 // CHECK:    cir.br ^bb[[#BLK8:]]
 // CHECK:  ^bb[[#BLK8]]:
-// CHECK:    cir.brcond {{.*}} ^bb[[#BLK9:]], ^bb[[#BLK10:]]
+// CHECK:    cir.br ^bb[[#BLK9:]]
 // CHECK:  ^bb[[#BLK9]]:
-// CHECK:    cir.br ^bb[[#RETURN:]]
+// CHECK:    cir.brcond {{.*}} ^bb[[#BLK10:]], ^bb[[#BLK11:]]
 // CHECK:  ^bb[[#BLK10]]:
-// CHECK:    cir.br ^bb[[#BLK11:]]
+// CHECK:    cir.br ^bb[[#RETURN:]]
 // CHECK:  ^bb[[#BLK11]]:
+// CHECK:    cir.br ^bb[[#BLK12:]]
+// CHECK:  ^bb[[#BLK12]]:
 // CHECK:    cir.br ^bb[[#LABEL_LOOP]]
 // CHECK:  ^bb[[#LABEL_LOOP]]:
-// CHECK:    cir.br ^bb[[#COND]]
-// CHECK:  ^bb[[#EXIT]]:
 // CHECK:    cir.br ^bb[[#BLK14:]]
 // CHECK:  ^bb[[#BLK14]]:
+// CHECK:    cir.br ^bb[[#COND]]
+// CHECK:  ^bb[[#EXIT]]:
+// CHECK:    cir.br ^bb[[#BLK16:]]
+// CHECK:  ^bb[[#BLK16]]:
 // CHECK:    cir.br ^bb[[#RETURN]]
 // CHECK:  ^bb[[#RETURN]]:
 // CHECK:    cir.return
@@ -290,8 +305,10 @@ void foo() {
 
 // NOFLAT: cir.func dso_local @_Z3foov()
 // NOFLAT:   cir.scope {
-// NOFLAT:     cir.label "label"
 // NOFLAT:     %0 = cir.alloca !rec_S, !cir.ptr<!rec_S>, ["agg.tmp0"]
+// NOFLAT:     cir.br ^bb1
+// NOFLAT:    ^bb1:
+// NOFLAT:     cir.label "label"
 
 extern "C" void action1();
 extern "C" void action2();

clang/test/CIR/CodeGen/label-values.c

@@ -41,16 +41,18 @@ void B(void) {
 }
 
 // CIR:  cir.func dso_local @B()
-// CIR:    cir.label "B"
 // CIR:    [[PTR:%.*]] = cir.alloca !cir.ptr<!void>, !cir.ptr<!cir.ptr<!void>>, ["ptr", init] {alignment = 8 : i64}
+// CIR:    cir.br ^bb1
+// CIR:   ^bb1:  // pred: ^bb0
+// CIR:    cir.label "B"
 // CIR:    [[BLOCK:%.*]] = cir.blockaddress <@B, "B"> -> !cir.ptr<!void>
 // CIR:    cir.store align(8) [[BLOCK]], [[PTR]] : !cir.ptr<!void>, !cir.ptr<!cir.ptr<!void>>
 // CIR:    cir.return
 
 // LLVM: define dso_local void @B
+// LLVM:   %[[PTR:.*]] = alloca ptr, i64 1, align 8
 // LLVM:   br label %[[B:.*]]
 // LLVM: [[B]]:
-// LLVM:   %[[PTR:.*]] = alloca ptr, i64 1, align 8
 // LLVM:   store ptr blockaddress(@B, %[[B]]), ptr %[[PTR]], align 8
 // LLVM:   ret void