[flang] handle allocation of zero-sized objects

Author: kkwli

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -1119,9 +1119,19 @@ struct AllocMemOpConversion : public fir::FIROpConversion<fir::AllocMemOp> {
     mlir::Value size = genTypeSizeInBytes(loc, ity, rewriter, llvmObjectTy);
     if (auto scaleSize = genAllocationScaleSize(heap, ity, rewriter))
       size = rewriter.create<mlir::LLVM::MulOp>(loc, ity, size, scaleSize);
-    for (mlir::Value opnd : adaptor.getOperands())
-      size = rewriter.create<mlir::LLVM::MulOp>(
-          loc, ity, size, integerCast(loc, rewriter, ity, opnd));
+    for (mlir::Value opnd : adaptor.getOperands()) {
+      auto arg = integerCast(loc, rewriter, ity, opnd);
+      auto val = fir::getIntIfConstant(arg);
+      if (val && *val == 0) {
+        // As the return value of malloc(0) is implementation defined, allocate
+        // one byte to ensure the allocation status being true. This behavior
+        // aligns to what the runtime has.
+        size = genConstantIndex(loc, ity, rewriter, 1);
+        break;
+      } else {
+        size = rewriter.create<mlir::LLVM::MulOp>(loc, ity, size, arg);
+      }
+    }
     auto mallocTyWidth = lowerTy().getIndexTypeBitwidth();
     auto mallocTy =
         mlir::IntegerType::get(rewriter.getContext(), mallocTyWidth);

flang/test/Lower/zero-size2.f90

@@ -0,0 +1,37 @@
+! RUN: %flang_fc1 -emit-llvm -o - %s | FileCheck %s
+
+subroutine sub1()
+  integer, allocatable :: arr(:)
+  allocate(arr(0))
+! CHECK-LABEL: @sub1_
+! CHECK: %[[p:.*]] = call ptr @malloc(i64 1)
+end
+
+subroutine sub2()
+  real, allocatable :: arr(:,:)
+  allocate(arr(10,0))
+! CHECK-LABEL: @sub2_
+! CHECK: %[[p:.*]] = call ptr @malloc(i64 1)
+end
+
+subroutine sub3(i)
+  integer :: i
+  real, allocatable :: arr(:,:)
+  allocate(arr(i,0))
+! CHECK-LABEL: @sub3_
+! CHECK: %[[p:.*]] = call ptr @malloc(i64 1)
+end
+
+subroutine sub4()
+  character(:), allocatable :: c
+  allocate(character(0)::c)
+! CHECK-LABEL: @sub4_
+! CHECK: %[[p:.*]] = call ptr @malloc(i64 1)
+end  
+
+subroutine sub5()
+  character(:), allocatable :: c(:)
+  allocate(character(5)::c(0))
+! CHECK-LABEL: @sub5_
+! CHECK: %[[p:.*]] = call ptr @malloc(i64 1)
+end