[flang] Add loop annotation attributes to the loop backedge (#126082)

Flang currently adds loop metadata to a conditional branch in the loop
preheader, while clang adds it to the loop latch's branch instruction.
Langref says:

> Currently, loop metadata is implemented as metadata attached to the
branch instruction in the loop latch block.
>
> https://llvm.org/docs/LangRef.html#llvm-loop

I misread langref a couple times, but I think this is the appropriate
branch op for the LoopAnnotationAttr. In a couple examples I found that
the metadata was lost entirely during canonicalization. This patch makes
the codegen look more like clang's and the annotations persist through
codegen.

* current clang: https://godbolt.org/z/8WhbcrnG3
* current flang: https://godbolt.org/z/TrPboqqcn

Author: ashermancinelli

flang/lib/Optimizer/Transforms/ControlFlowConverter.cpp

@@ -123,23 +123,24 @@ class CfgLoopConv : public mlir::OpRewritePattern<fir::DoLoopOp> {
                                       : terminator->operand_begin();
     loopCarried.append(begin, terminator->operand_end());
     loopCarried.push_back(itersMinusOne);
-    rewriter.create<mlir::cf::BranchOp>(loc, conditionalBlock, loopCarried);
+    auto backEdge =
+        rewriter.create<mlir::cf::BranchOp>(loc, conditionalBlock, loopCarried);
     rewriter.eraseOp(terminator);
 
+    // Copy loop annotations from the do loop to the loop back edge.
+    if (auto ann = loop.getLoopAnnotation())
+      backEdge->setAttr("loop_annotation", *ann);
+
     // Conditional block
     rewriter.setInsertionPointToEnd(conditionalBlock);
     auto zero = rewriter.create<mlir::arith::ConstantIndexOp>(loc, 0);
     auto comparison = rewriter.create<mlir::arith::CmpIOp>(
         loc, arith::CmpIPredicate::sgt, itersLeft, zero);
 
-    auto cond = rewriter.create<mlir::cf::CondBranchOp>(
+    rewriter.create<mlir::cf::CondBranchOp>(
         loc, comparison, firstBlock, llvm::ArrayRef<mlir::Value>(), endBlock,
         llvm::ArrayRef<mlir::Value>());
 
-    // Copy loop annotations from the do loop to the loop entry condition.
-    if (auto ann = loop.getLoopAnnotation())
-      cond->setAttr("loop_annotation", *ann);
-
     // The result of the loop operation is the values of the condition block
     // arguments except the induction variable on the last iteration.
     auto args = loop.getFinalValue()

flang/test/Fir/vector-always.fir

@@ -13,7 +13,9 @@ func.func @_QPvector_always() -> i32 {
     %c10_i32 = arith.constant 10 : i32
     %c1_i32 = arith.constant 1 : i32
     %c10 = arith.constant 10 : index
-// CHECK:   cf.cond_br %{{.*}}, ^{{.*}}, ^{{.*}} {loop_annotation = #[[ANNOTATION]]}
+// CHECK: cf.cond_br
+// CHECK-NOT: loop_annotation
+// CHECK:   cf.br ^{{.*}} {loop_annotation = #[[ANNOTATION]]}
     %8:2 = fir.do_loop %arg0 = %c1 to %c10 step %c1 iter_args(%arg1 = %c1_i32) -> (index, i32) attributes {loopAnnotation = #loop_annotation} {
       fir.result %c1, %c1_i32 : index, i32
     }

flang/test/Integration/unroll.f90

@@ -3,8 +3,10 @@
 ! CHECK-LABEL: unroll_dir
 subroutine unroll_dir
   integer :: a(10)
-  !dir$ unroll
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[UNROLL_ENABLE_FULL_ANNO:.*]]
+  !dir$ unroll 
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[UNROLL_ENABLE_FULL_ANNO:.*]]
   do i=1,10
   a(i)=i
   end do
@@ -14,7 +16,9 @@ end subroutine unroll_dir
 subroutine unroll_dir_0
   integer :: a(10)
   !dir$ unroll 0
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[UNROLL_DISABLE_ANNO:.*]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[UNROLL_DISABLE_ANNO:.*]]
   do i=1,10
   a(i)=i
   end do
@@ -24,7 +28,9 @@ end subroutine unroll_dir_0
 subroutine unroll_dir_1
   integer :: a(10)
   !dir$ unroll 1
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[UNROLL_DISABLE_ANNO]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[UNROLL_DISABLE_ANNO]]
   do i=1,10
   a(i)=i
   end do
@@ -34,7 +40,9 @@ end subroutine unroll_dir_1
 subroutine unroll_dir_2
   integer :: a(10)
   !dir$ unroll 2
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[UNROLL_ENABLE_COUNT_2:.*]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[UNROLL_ENABLE_COUNT_2:.*]]
   do i=1,10
   a(i)=i
   end do

flang/test/Integration/unroll_and_jam.f90

@@ -4,7 +4,9 @@
 subroutine unroll_and_jam_dir
   integer :: a(10)
   !dir$ unroll_and_jam 4
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[ANNOTATION:.*]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[ANNOTATION:.*]]
   do i=1,10
      a(i)=i
   end do
@@ -14,7 +16,9 @@ end subroutine unroll_and_jam_dir
 subroutine unroll_and_jam_dir_0
   integer :: a(10)
   !dir$ unroll_and_jam 0
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[ANNOTATION_DISABLE:.*]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[ANNOTATION_DISABLE:.*]]
   do i=1,10
   a(i)=i
   end do
@@ -24,7 +28,9 @@ end subroutine unroll_and_jam_dir_0
 subroutine unroll_and_jam_dir_1
   integer :: a(10)
   !dir$ unroll_and_jam 1 
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[ANNOTATION_DISABLE]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[ANNOTATION_DISABLE]]
   do i=1,10
   a(i)=i
   end do
@@ -34,7 +40,9 @@ end subroutine unroll_and_jam_dir_1
 subroutine nounroll_and_jam_dir
   integer :: a(10)
   !dir$ nounroll_and_jam
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[ANNOTATION_DISABLE]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[ANNOTATION_DISABLE]]
   do i=1,10
   a(i)=i
   end do
@@ -44,7 +52,9 @@ end subroutine nounroll_and_jam_dir
 subroutine unroll_and_jam_dir_no_factor
   integer :: a(10)
   !dir$ unroll_and_jam
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[ANNOTATION_NO_FACTOR:.*]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[ANNOTATION_NO_FACTOR:.*]]
   do i=1,10
   a(i)=i
   end do

flang/test/Integration/vector-always.f90

@@ -4,7 +4,9 @@
 subroutine vector_always
   integer :: a(10)
   !dir$ vector always
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[ANNOTATION:.*]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[ANNOTATION:.*]]
   do i=1,10
      a(i)=i
   end do
@@ -14,7 +16,9 @@ end subroutine vector_always
 subroutine no_vector
   integer :: a(10)
   !dir$ novector
-  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}, !llvm.loop ![[ANNOTATION2:.*]]
+  ! CHECK:   br i1 {{.*}}, label {{.*}}, label {{.*}}
+  ! CHECK-NOT: !llvm.loop
+  ! CHECK:   br label {{.*}}, !llvm.loop ![[ANNOTATION2:.*]]
   do i=1,10
      a(i)=i
   end do