[flang] Do not error on constant nonzero UB in substring of ZLA

[tmjbios]

A substring reference where the lower bound is higher than the upper bound is defined in 9.4.1 to be zero-length.

Thus, a reference to a substring of a CHARACTER*(0) string such as

string(foo():2)

cannot be a compile-time error since we do not know the return value of foo().

We also should not error if the lbound > ubound at compile time.

[llvmbot]

@llvm/pr-subscribers-flang-semantics

Author: TMJ (tmjbios)

Changes

A substring reference where the lower bound is higher than the upper bound is defined in 9.4.1 to be zero-length.

Thus, a reference to a substring of a CHARACTER*(0) string such as

string(foo():2)

cannot be a compile-time error since we do not know the return value of foo().

We also should not error if the lbound > ubound at compile time.

---

Full diff: https://github.com/llvm/llvm-project/pull/174511.diff

2 Files Affected:

• (modified) flang/lib/Semantics/expression.cpp (+2-2)
• (added) flang/test/Semantics/zero_len_char_array.f90 (+30)

diff --git a/flang/lib/Semantics/expression.cpp b/flang/lib/Semantics/expression.cpp
index 402e88252fc20..89bc7264707de 100644
--- a/flang/lib/Semantics/expression.cpp
+++ b/flang/lib/Semantics/expression.cpp
@@ -1221,9 +1221,9 @@ MaybeExpr ExpressionAnalyzer::Analyze(const parser::Substring &ss) {
                 Say("Substring must begin at 1 or later, not %jd"_err_en_US,
                     static_cast<std::intmax_t>(*lbValue));
                 return std::nullopt;
-              } else if (ubValue && len && *ubValue > *len &&
+              } else if (ubValue && len && *len > 0 && *ubValue > *len &&
                   (lbValue || !first)) {
-                Say("Substring must end at %zd or earlier, not %jd"_err_en_US,
+                Say("Substring must end at %jd or earlier, not %jd"_err_en_US,
                     static_cast<std::intmax_t>(*len),
                     static_cast<std::intmax_t>(*ubValue));
                 return std::nullopt;
diff --git a/flang/test/Semantics/zero_len_char_array.f90 b/flang/test/Semantics/zero_len_char_array.f90
new file mode 100644
index 0000000000000..ee9f10fdd6d67
--- /dev/null
+++ b/flang/test/Semantics/zero_len_char_array.f90
@@ -0,0 +1,30 @@
+! RUN: %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck --allow-empty %s
+! CHECK-NOT: error:
+! Test that zero-length character substrings with lbound>ubounds indices
+! do not produce spurious errors when the character array is zero length.
+
+integer function init(i)
+  integer i
+  init=i
+end
+
+program flang_t7052
+  character*(*), parameter :: param_char = ""
+  character*(0)            :: zero_len_char
+
+  if ( param_char(init(5):init(3)) > zero_len_char(1:-2) ) then
+    print *,"Test failed"
+  endif
+
+  if ( param_char(init(5):init(3)) > zero_len_char(10:2) ) then
+    print *,"Test failed"
+  endif
+
+  if ( param_char(init(5):init(3)) > zero_len_char(init(10):2) ) then
+    print *,"Test failed"
+  endif
+
+  if ( param_char(init(5):init(3)) > zero_len_char(init(10):-2) ) then
+    print *,"Test failed"
+  endif
+end program

[clementval]

Please add a prefix to your commit message so we know it is related to flang ([flang])

[tmjbios]

Bah! Sorry about that, Val.

[akuhlens]

This looks great to me.

[eugeneepshteyn]

Thank you for the PR.

Could you please also run Fortran tests from llvm-test-suite?

[eugeneepshteyn]

Thank you for the changes. LGTM!

[tmjbios]

Thanks!

I don't have permission yet to merge my own PRs, so I'll need someone else to squash & merge.

[eugeneepshteyn]

@tmjbios , did you run Fortran tests from llvm-test-suite with this change?

[tmjbios]

Yes. I'm still getting used to the various test tools and methods for Flang; in addition to our internal test suite for CCE (where applicable) and the flang "make" lit test targets for flang and flang-rt, I follow this link's instructions for the llvm-test-suite for everything I'm doing:

https://github.com/llvm/llvm-test-suite/blob/main/Fortran/gfortran/README.md#usage
https://github.com/llvm/llvm-project/blob/main/flang/docs/FortranLLVMTestSuite.md

If there is additional pre-commit testing you would like to see us doing religiously, please let me know.