jcranmer-intel added inline comments.
================ Comment at: clang/lib/AST/ExprConstant.cpp:12452 + int Ilogb; + if (APFloat::opStatus St = ilogb(F, Ilogb); !isConstantOpStatus(St)) + return false; ---------------- Izaron wrote: > aaron.ballman wrote: > > jcranmer-intel wrote: > > > `long double` is `ppc_fp128` on at least some PPC targets, and while I'm > > > not entirely certain of what `ilogb` properly returns in the corner cases > > > of the `ppc_fp128`, I'm not entirely confident that the implementation of > > > `APFloat` is correct in those cases. I'd like someone with PPC background > > > to comment in here. > > Paging @hubert.reinterpretcast for help finding a good person to comment on > > the PPC questions. > @jcranmer-intel constexpr evaluation is quite machine-/target-independent. > Clang evaluates it based on its **internal** representation of float > variables. > > [[ > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/llvm/include/llvm/ADT/APFloat.h#L1256 > | int ilogb ]] uses `Arg.getIEEE()`, that [[ > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/llvm/include/llvm/ADT/APFloat.h#L819-L825 > | returns Clang's internal float representation ]]. > > Whichever float semantics is being used, [[ > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/llvm/lib/Support/APFloat.cpp#L54-L61 > | minExponent and maxExponent are representable as APFloatBase::ExponentType > ]], which is `int32_t`: > ``` > /// A signed type to represent a floating point numbers unbiased exponent. > typedef int32_t ExponentType; > ``` > > We already use `int ilogb` in some constexpr evaluation code: [[ > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/clang/lib/AST/ExprConstant.cpp#L14592 > | link ]], it is working correct because it is working on Clang's float > representations. > We already use `int ilogb` in some constexpr evaluation code: [[ > https://github.com/llvm/llvm-project/blob/2e5bf4da99a2f8d3d4bb4f1a4d1ed968a01e8f02/clang/lib/AST/ExprConstant.cpp#L14592 > | link ]], it is working correct because it is working on Clang's float > representations. `APFloat::getIEEE()`, if I'm following it correctly, only returns the details of the high double in `ppc_fp128` floats, and I'm not sufficiently well-versed in the `ppc_fp128` format to establish whether or not the low double comes into play here. glibc seems to think that the low double comes into play in at least one case: https://github.com/bminor/glibc/blob/30891f35fa7da832b66d80d0807610df361851f3/sysdeps/ieee754/ldbl-128ibm/e_ilogbl.c Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D136568/new/ https://reviews.llvm.org/D136568 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
