[llvm-bugs] [Bug 45593] New: bad literal and implicit cast inserted for aarch32 unsigned long literal

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45593

            Bug ID: 45593
           Summary: bad literal and implicit cast inserted for aarch32
                    unsigned long literal
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ndesaulniers at google.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, kristof.beyls at arm.com,
                    llozano at chromium.org, llvm-bugs at lists.llvm.org,
                    natechancellor at gmail.com, oliver.stannard at arm.com,
                    richard-llvm at metafoo.co.uk, srhines at google.com
            Blocks: 4068

https://godbolt.org/z/ZNBWZN is a curious case, reported by Nathan Chancellor
when looking into warning on the 5.4 LTS Linux kernel for ARCH=arm.

It seems that for
```
void foo(void) {
    unsigned long bar = 0x5346434e49ul;
}
```
$ clang -Wconstant-conversion --target=arm-linux-gnueabi foo.c
<source>:2:25: warning: implicit conversion from 'unsigned long long' to
'unsigned long' changes value from 357661101641 to 1178816073
[-Wconstant-conversion]

    unsigned long bar = 0x5346434e49ul;

                  ~~~   ^~~~~~~~~~~~~~

which is unusual, and seems specific to the 32b arm target.

It seems the AST is inserting the implicit cast:
TranslationUnitDecl
`-FunctionDecl <line:1:1, line:3:1> line:1:6 foo 'void (void)'
  `-CompoundStmt <col:16, line:3:1>
    `-DeclStmt <line:2:5, col:39>
      `-VarDecl <col:5, col:25> col:19 bar 'unsigned long' cinit
        `-ImplicitCastExpr <col:25> 'unsigned long' <IntegralCast>
          `-IntegerLiteral <col:25> 'unsigned long long' 357661101641

The IntegerLiteral being interpreted as 'unsigned long long' is unexpected, and
the ImplicitCastExpr to 'unsigned long' is unexpected, and triggering the
warning.

Removing the target argument shows the ImplicitCastExpr node removed, and the
IntegerLiteral interpreted as 'unsigned long' (as expected due to the `ul`
suffix). example:

TranslationUnitDecl
`-FunctionDecl <line:1:1, line:3:1> line:1:6 foo 'void (void)'
  `-CompoundStmt <col:16, line:3:1>
    `-DeclStmt <line:2:5, col:39>
      `-VarDecl <col:5, col:25> col:19 bar 'unsigned long' cinit
        `-IntegerLiteral <col:25> 'unsigned long' 357661101641


Referenced Bugs:

https://bugs.llvm.org/show_bug.cgi?id=4068
[Bug 4068] [Meta] Compiling the Linux kernel with clang
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