rnk added a comment. Let me know if it would be more helpful to set up a call, that might help us reach agreement sooner. I've used discord for this previously if that works for you, my username there is the same (`@rnk#8591`).
In D115441#3194066 <https://reviews.llvm.org/D115441#3194066>, @pengfei wrote: >> GCC doesn't align fp80 long double to 16 bytes on i686, so I see no reason >> for LLVM to do it. Is there some other compiler that you need ABI >> compatibility with? > > Yes. ICC aligns long double to 16 bytes on 32bit Windows. (I mentioned it in > the summary :). In contrast with GCC, ICC is more compatible with MSVC. So I > think it's reasonable to align with ICC ranther than GCC. My comment here refers to the alignment of argument values, not user-declared variables. The frontend controls the alignment of user-declared variables by setting the alloca alignment. GCC and ICC appear to align long double arguments to 4 bytes: https://gcc.godbolt.org/z/PbobWdrPf >> Also consider that in LLVM, the alignment of arguments passed in memory is >> not observable (unless byval or inalloca is used). If the user takes the >> address of an argument, they actually take the address of a local alloca, >> which is a copy of the argument. The frontend (clang) decides the alignment >> of the alloca. > > We cannot force user to always pass arguments by address. Once they are > passed by value, (actually it's common in the code, we usually write like > foo(double a) rather than foo(double *a) ), it turns to the scope of calling > conversion. It's true all basic types except f80 is aligned to 4 when passed > by value on 32 bits. But Windows 32 bits is not alone. Darwin 32 bits uses > the same calling conversion for f80 too. I don't intend to ask users to pass arguments by address. What I mean is, LLVM will copy an under-aligned long double argument passed in memory to properly aligned memory. Consider this example using double: https://gcc.godbolt.org/z/v6oTfrTr5 void escape(void*); void foo(int x, double d, int y) { escape(&x); escape(&d); escape(&y); } --> "?foo@@YAXHNH@Z": # @"?foo@@YAXHNH@Z" push ebp mov ebp, esp # Align stack to 8 bytes and esp, -8 sub esp, 8 # Copy bytes of d to aligned memory movsd xmm0, qword ptr [ebp + 12] # xmm0 = mem[0],zero movsd qword ptr [esp], xmm0 lea eax, [ebp + 8] ... # Take address of aligned alloca at ESP+0 mov eax, esp push eax call "?escape@@YAXPAX@Z" --- So, I'm still not convinced we have to change the LLVM data layout. Is there some other aspect of calling convention lowering that needs to know the alignment of long double? Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D115441/new/ https://reviews.llvm.org/D115441 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
