[clang] [flang] [lldb] [lld] [libcxx] [compiler-rt] [llvm] [clang-tools-extra] [libc] Fix clang to recognize new C23 modifiers %w and %wf when printing (PR #71771)

[via llvm-commits]

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@@ -286,7 +286,33 @@ clang::analyze_format_string::ParseLengthModifier(FormatSpecifier &FS,
       lmKind = LengthModifier::AsInt3264;
       break;
     case 'w':
-      lmKind = LengthModifier::AsWide; ++I; break;
+      ++I;
+      if (I == E) return false;
+      if (*I == 'f') {
+        lmKind = LengthModifier::AsWideFast;
+        ++I;
+      } else {
+        lmKind = LengthModifier::AsWide;
+      }
+
+      if (I == E) return false;
+      int s = 0;
+      while (unsigned(*I - '0') <= 9) {
+        s = 10 * s + unsigned(*I - '0');
+        ++I;
+      }
+
+      // s == 0 is MSVCRT case, like l but only for c, C, s, S, or Z on windows
+      // s != 0 for b, d, i, o, u, x, or X when a size followed(like 8, 16, 32 or 64)
+      if (s != 0) {
+        std::set<int> supported_list {8, 16, 32, 64};
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enh-google wrote:

> So I think we should probably err on the side of specifying all the bit-widths we specify in stdint.h.

as a libc maintainer (who happens to have done a survey of the other libcs on this specific bit of C23 functionality, when zijunzhao was implementing it for bionic :-) ), i'd argue the opposite: none of bionic, glibc, musl, FreeBSD, and Apple's fork of FreeBSD libc supports these weird sizes[1]. nor does any hardware i'm aware of. i'd actually argue that the llvm stdint.h change that added these types should be reverted[2]. (presumably someone who knows the llvm code better can check whether it's possible for clang to ever actually define `__INT48_TYPE__` and its non-power-of-two friends? if there really _is_ such an architecture, we could at least get a useful code comment in stdint.h out of it!)

as for the diagnostics, i'd argue (a) it doesn't make sense having this be libc-specific (like, for example, the existing "do math functions set errno?" configuration) since every libc in use would have the same "no, we don't support 48-bit ints" setting and (b) saying "well, 56-bit ints _might_ be a thing in theory, so we'll punt and leave it to be runtime error" isn't very helpful in a world where it will always be a runtime error.

___
1. to be fair, a couple of them still don't implement %w at all. at the risk of making more work for zijunzhao, if you were going to teach clang about different libc versions, _that_ would at least be useful (for those targets that include a version in them): "which version of Android/iOS first had %b?" etc. if i'm using %b but targeting a version that didn't have it, that's a useful compile-time warning, at least as long as anyone's targeting old-enough versions. (and, full disclosure: for Android that's the same as %w: they're both new in this year's release --- https://android.googlesource.com/platform/bionic/+/HEAD/docs/status.md)
2. personally, i don't feel like the commit message on the change that introduced this stuff to stdint.h motivated it at all. i suspect if llvm hadn't still been an academic project back then, that change would never have been accepted in the first place!

https://github.com/llvm/llvm-project/pull/71771