[clang] incorrect code generation when building gawk 5.2.1 using -O2/-O3

[0xC0ncord]

It was found that when building gawk 5.2.1 with -O2 or -O3, incorrect machine code is generated which is causing differing runtime behavior than expected.

Background

Originally I ran into this when upgrading gawk on my personal Gentoo systems to 5.2.1. There is a particular regex[1] used in plymouth which was suddenly erroring that did not previously. Downgrading to gawk 5.1.1, building it with gcc, or disabling compiler optimizations would work around the issue.

This issue is being submitted after some discussion[2] on the gawk-bug mailing list, particularly because of these findings[3].

A simple test case is this command:

head /dev/zero | awk 'BEGIN { RS="[[][:blank:]]" }'

When running this command, the expected behavior is no output and a clean exit. However, when building gawk 5.2.1 with clang and -O2 or -O3, we see this error (and a non-zero exit code):

awk: cmd. line:1: fatal: invalid character class

[1] https://gitlab.freedesktop.org/plymouth/plymouth/-/blob/main/scripts/plymouth-set-default-theme.in#L50
[2] https://lists.gnu.org/archive/html/bug-gawk/2022-12/msg00010.html
[3] https://lists.gnu.org/archive/html/bug-gnulib/2023-01/msg00002.html

[thesamesam]

cc @eggert

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[MaskRay]

As noticed, gawk/support/dfa.c lines 1141-1143 confuses Clang. Clang thinks the ConditionalOperator (... ? dfaerror : dfawarn) is noreturn as dfaerror is noreturn (while it shouldn't since dfawarn isn't noreturn).

    ((dfa->syntax.dfaopts & DFA_CONFUSING_BRACKETS_ERROR
      ? dfaerror : dfawarn)
     (_("character class syntax is [[:space:]], not [:space:]")));

This problem has been known since 2010 (8c6b56f)

QualType ASTContext::mergeFunctionTypes(QualType lhs, QualType rhs, ...
...
  // It's noreturn if either type is.
  // FIXME: some uses, e.g. conditional exprs, really want this to be 'both'.
  bool NoReturn = lbaseInfo.getNoReturn() || rbaseInfo.getNoReturn();

In some contexts (merging two declarations), we do want to use the logical OR result. But here for the ConditionalOperator, we want to use the logical AND result.

[MaskRay]

Patch: https://reviews.llvm.org/D140868

[llvmbot]

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