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[clang][ExprConstant] Reject integral casts of addr-label-diffs... #171437

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[clang][ExprConstant] Reject integral casts of addr-label-diffs... #171437

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9 changes: 6 additions & 3 deletions clang/lib/AST/ExprConstant.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -18606,12 +18606,15 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {

if (!Result.isInt()) {
// Allow casts of address-of-label differences if they are no-ops
// or narrowing. (The narrowing case isn't actually guaranteed to
// or narrowing, if the result is at least 32 bits wide.
// (The narrowing case isn't actually guaranteed to
// be constant-evaluatable except in some narrow cases which are hard
// to detect here. We let it through on the assumption the user knows
// what they are doing.)
if (Result.isAddrLabelDiff())
return Info.Ctx.getTypeSize(DestType) <= Info.Ctx.getTypeSize(SrcType);
if (Result.isAddrLabelDiff()) {
unsigned DestBits = Info.Ctx.getTypeSize(DestType);
return DestBits >= 32 && DestBits <= Info.Ctx.getTypeSize(SrcType);
}
// Only allow casts of lvalues if they are lossless.
return Info.Ctx.getTypeSize(DestType) == Info.Ctx.getTypeSize(SrcType);
}
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2 changes: 1 addition & 1 deletion clang/test/AST/ast-dump-APValue-addrlabeldiff.c
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Expand Up @@ -12,7 +12,7 @@

// CHECK: | |-value: AddrLabelDiff &&l2 - &&l1
int Test(void) {
constexpr char ar = &&l2 - &&l1;
constexpr long long ar = &&l2 - &&l1;
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@AaronBallman AaronBallman Dec 10, 2025

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Can we keep a test where the result is narrowed to show that we reject those, or is that already covered elsewhere?

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@tbaederr tbaederr Dec 11, 2025

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Yes, in clang/test/CodeGen/const-label-addr.c for example:

int b(void) {
  static int ar = &&l2 - &&l1;
l1:
  return 10;
l2:
  return 11;
}

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@AaronBallman AaronBallman Dec 11, 2025

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That's not what I was after, I meant a Sema test showing we diagnose:

int b(void) {
  static unsigned char ar = &&l2 - &&l1; // error
l1:
  return 10;
l2:
  return 11;
}

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@tbaederr tbaederr Dec 11, 2025

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Oh, I guess we don't?
We also don't reject widening casts in sema today:

int b(void) {
  static unsigned __int128 ar = &&l2 - &&l1; // error
l1:
  return 10;
l2:
  return 11;
}

but we don't constant fold them.

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@AaronBallman AaronBallman Dec 11, 2025

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I was basing this off the patch description saying: Reject integral casts of addr-label-diffs if the result is narrower than 32 bits. ;-)

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@tbaederr tbaederr Dec 12, 2025

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This patch rejects those casts in the same way we've always rejected widening casts :P

l1:
return 10;
l2:
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4 changes: 4 additions & 0 deletions clang/test/CodeGenCXX/const-init.cpp
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Expand Up @@ -73,6 +73,10 @@ __int128_t PR11705 = (__int128_t)&PR11705;
// CHECK: @_ZZ23UnfoldableAddrLabelDiffvE1x = internal global i128 0
void UnfoldableAddrLabelDiff() { static __int128_t x = (long)&&a-(long)&&b; a:b:return;}

// CHECK: @_ZZ24UnfoldableAddrLabelDiff2vE1x = internal global i16 0
void UnfoldableAddrLabelDiff2() { static short x = (long)&&a-(long)&&b; a:b:return;}


// But make sure we do fold this.
// CHECK: @_ZZ21FoldableAddrLabelDiffvE1x = internal global i64 sub (i64 ptrtoint (ptr blockaddress(@_Z21FoldableAddrLabelDiffv
void FoldableAddrLabelDiff() { static long x = (long)&&a-(long)&&b; a:b:return;}
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