[flang-commits] [flang] [flang] Fix length handling in character kind implicit conversion (PR #74586)

[via flang-commits]

https://github.com/jeanPerier created https://github.com/llvm/llvm-project/pull/74586

When assigning to a whole allocatable, lowering is dealing with the implicit conversion to preserve the RHS lower bounds. In case of character KIND mismatch, the code was setting the new RHS length to the one from the LHS, which is wrong for two reasons:
- no padding/truncation was actually done in the conversion
- the RHS length should anyway not be touched since the one from the allocatable LHS may change to become the one of the RHS.

Update the code to preserve the RHS type length when materializing the implicit character KIND conversion.

>From 4069902edab88bfa7ed03327d8fd704f9028fe40 Mon Sep 17 00:00:00 2001
From: Jean Perier <jperier at nvidia.com>
Date: Wed, 6 Dec 2023 02:55:50 -0800
Subject: [PATCH] [flang] Fix length handling in character kind implicit
 conversion

When assigning to a whole allocatable, lowering is dealing with the
implicit conversion to preserve the RHS lower bounds.
In case of character KIND mismatch, the code was setting the new RHS
length to the one from the LHS, which is wrong for two reasons:
- no padding/truncation was actually done in the conversion
- the RHS length should not be touched since the one from the
  allocatable LHS may change to become the one of the RHS.

Update the code to preserve the RHS type length.
---
 flang/lib/Optimizer/Builder/HLFIRTools.cpp    |  7 ++++-
 .../implicit-type-conversion-allocatable.f90  | 26 +++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/flang/lib/Optimizer/Builder/HLFIRTools.cpp b/flang/lib/Optimizer/Builder/HLFIRTools.cpp
index 0b1e57e8f6c37..17efa45b8667d 100644
--- a/flang/lib/Optimizer/Builder/HLFIRTools.cpp
+++ b/flang/lib/Optimizer/Builder/HLFIRTools.cpp
@@ -1132,8 +1132,13 @@ hlfir::genTypeAndKindConvert(mlir::Location loc, fir::FirOpBuilder &builder,
   std::optional<int> toKindCharConvert;
   if (auto toCharTy = mlir::dyn_cast<fir::CharacterType>(toType)) {
     if (auto fromCharTy = mlir::dyn_cast<fir::CharacterType>(fromType))
-      if (toCharTy.getFKind() != fromCharTy.getFKind())
+      if (toCharTy.getFKind() != fromCharTy.getFKind()) {
         toKindCharConvert = toCharTy.getFKind();
+        // Preserve source length (padding/truncation will occur in assignment
+        // if needed).
+        toType = fir::CharacterType::get(
+            fromType.getContext(), toCharTy.getFKind(), fromCharTy.getLen());
+      }
     // Do not convert in case of character length mismatch only, hlfir.assign
     // deals with it.
     if (!toKindCharConvert)
diff --git a/flang/test/Lower/HLFIR/implicit-type-conversion-allocatable.f90 b/flang/test/Lower/HLFIR/implicit-type-conversion-allocatable.f90
index 7083a825dfd3b..361cd61adea2d 100644
--- a/flang/test/Lower/HLFIR/implicit-type-conversion-allocatable.f90
+++ b/flang/test/Lower/HLFIR/implicit-type-conversion-allocatable.f90
@@ -38,3 +38,29 @@ subroutine preserve_lbounds(x, y)
 ! CHECK:           hlfir.destroy %[[VAL_8]] : !hlfir.expr<?xi32>
 ! CHECK:           return
 ! CHECK:         }
+
+! Test that RHS character length is preserved in a character KIND
+! conversion before the assignment.
+subroutine kind_and_length(a, b)
+  character(len=4,kind=4), allocatable :: a(:)
+  character(len=2,kind=1) :: b(:)
+  a = b
+end subroutine
+! CHECK-LABEL:   func.func @_QPkind_and_length(
+! CHECK:         %[[VAL_2:.*]] = arith.constant 4 : index
+! CHECK:         %[[VAL_3:.*]]:2 = hlfir.declare %[[VAL_0:[a-z0-9]*]] typeparams %[[VAL_2:[a-z0-9]*]]  {{.*}}Ea
+! CHECK:         %[[VAL_4:.*]] = arith.constant 2 : index
+! CHECK:         %[[VAL_5:.*]]:2 = hlfir.declare %[[VAL_1:[a-z0-9]*]] typeparams %[[VAL_4:[a-z0-9]*]]  {{.*}}Eb
+! CHECK:         %[[VAL_6:.*]] = arith.constant 0 : index
+! CHECK:         %[[VAL_7:.*]]:3 = fir.box_dims %[[VAL_5]]#0, %[[VAL_6]] : (!fir.box<!fir.array<?x!fir.char<1,2>>>, index) -> (index, index, index)
+! CHECK:         %[[VAL_8:.*]] = fir.shape %[[VAL_7]]#1 : (index) -> !fir.shape<1>
+! CHECK:         %[[VAL_9:.*]] = hlfir.elemental %[[VAL_8]] typeparams %[[VAL_4]] unordered : (!fir.shape<1>, index) -> !hlfir.expr<?x!fir.char<4,2>> {
+! CHECK:         ^bb0(%[[VAL_10:.*]]: index):
+! CHECK:           %[[VAL_11:.*]] = hlfir.designate %[[VAL_5]]#0 (%[[VAL_10]])  typeparams %[[VAL_4]] : (!fir.box<!fir.array<?x!fir.char<1,2>>>, index, index) -> !fir.ref<!fir.char<1,2>>
+! CHECK:           %[[VAL_12:.*]] = fir.alloca !fir.char<4,?>(%[[VAL_4]] : index)
+! CHECK:           fir.char_convert %[[VAL_11]] for %[[VAL_4]] to %[[VAL_12]] : !fir.ref<!fir.char<1,2>>, index, !fir.ref<!fir.char<4,?>>
+! CHECK:           %[[VAL_13:.*]]:2 = hlfir.declare %[[VAL_12:[a-z0-9]*]] typeparams %[[VAL_4:[a-z0-9]*]] {uniq_name = ".temp.kindconvert"} : (!fir.ref<!fir.char<4,?>>, index) -> (!fir.boxchar<4>, !fir.ref<!fir.char<4,?>>)
+! CHECK:           hlfir.yield_element %[[VAL_13]]#0 : !fir.boxchar<4>
+! CHECK:         }
+! CHECK:         hlfir.assign %[[VAL_9]] to %[[VAL_3]]#0 realloc keep_lhs_len : !hlfir.expr<?x!fir.char<4,2>>, !fir.ref<!fir.box<!fir.heap<!fir.array<?x!fir.char<4,4>>>>>
+