[flang-commits] [flang] 7c7a4f1 - [flang] fix do while lowering to SCF after unstructured context (#181996)

[via flang-commits]

Author: jeanPerier
Date: 2026-02-18T18:09:48+01:00
New Revision: 7c7a4f1cf55353a86bdf9b002139fbc8450650c3

URL: https://github.com/llvm/llvm-project/commit/7c7a4f1cf55353a86bdf9b002139fbc8450650c3
DIFF: https://github.com/llvm/llvm-project/commit/7c7a4f1cf55353a86bdf9b002139fbc8450650c3.diff

LOG: [flang] fix do while lowering to SCF after unstructured context (#181996)

`maybeStartBlock(preheaderBlock);` should not be called when lowering a
DO WHILE in a structured fashion, it is meant to insert branches in
unstructured contexts lowering.

This had the side effect of inserting a back edge to the block
containing the scf for DO WHILE after a construct lowered in an
unstructured fashion because blocks may be unterminated after
unstructured lowering, and `maybeStartBlock` created a `cf.br` to its
argument when the current block is unterminated (and here, the argument
and current block where the same, creating an infinite loop).

Added: 
    

Modified: 
    flang/lib/Lower/Bridge.cpp
    flang/test/Lower/do-while-to-scf-while.f90

Removed: 
    


################################################################################
diff  --git a/flang/lib/Lower/Bridge.cpp b/flang/lib/Lower/Bridge.cpp
index 6eedb089eac40..687c2f0f4a42a 100644
--- a/flang/lib/Lower/Bridge.cpp
+++ b/flang/lib/Lower/Bridge.cpp
@@ -2540,7 +2540,6 @@ class FirConverter : public Fortran::lower::AbstractConverter {
       // PFT branch analysis), allowing the loop to exit only when the condition
       // becomes false.
       if (!unstructuredContext) {
-        maybeStartBlock(preheaderBlock); // no block or empty block
         genDoWhileAsSCFWhile(*whileCondition, eval, doStmtEval);
         return;
       }

diff  --git a/flang/test/Lower/do-while-to-scf-while.f90 b/flang/test/Lower/do-while-to-scf-while.f90
index d2f38d6e09694..6d057ed823c36 100644
--- a/flang/test/Lower/do-while-to-scf-while.f90
+++ b/flang/test/Lower/do-while-to-scf-while.f90
@@ -1,4 +1,4 @@
-! RUN: bbc -emit-fir -hlfir=false -lower-do-while-to-scf-while %s -o - | FileCheck %s
+! RUN: bbc -emit-hlfir -lower-do-while-to-scf-while %s -o - | FileCheck %s
 
 ! CHECK-LABEL: func.func @_QPsimple_do_while()
 ! CHECK: scf.while
@@ -85,3 +85,20 @@ subroutine do_while_goto_internal_backedge()
   print *, "sum=", sum
 end subroutine do_while_goto_internal_backedge
 
+! CHECK-LABEL:   func.func @_QPtest_after_unstructured(
+! CHECK:  scf.while
+! CHECK-NOT: cf.br
+! CHECK: return
+subroutine test_after_unstructured(cdt, switch)
+  logical :: cdt, eval
+  integer :: switch, i = 1
+  if (cdt) then
+    select case (switch)
+      case (0)
+        call print1()
+    end select
+  end if
+  do while(eval(i))
+    call incr(i)
+  end do
+end subroutine