On Thu, Dec 03, 2020 at 03:28:14PM -0800, Nick Desaulniers wrote: > On Thu, Dec 3, 2020 at 3:22 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > > > On Tue, Nov 24, 2020 at 9:28 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > > > > > On Tue, Nov 03, 2020 at 04:53:43PM -0800, Nick Desaulniers wrote: > > > > DWARF v5 is the latest standard of the DWARF debug info format. > > > > > > > > Feature detection of DWARF5 is onerous, especially given that we've > > > > removed $(AS), so we must query $(CC) for DWARF5 assembler directive > > > > support. GNU `as` only recently gained support for specifying > > > > -gdwarf-5. > > > > > > With gcc, using -gdwarf-5 even without -Wa,--gdwarf-5 results in > > > considerably smaller debug info. gcc does not seem to generate the .file 0 > > > directive that causes older GNU as to barf. > > > > > > Should the assembler support check be restricted to CC_IS_CLANG? > > > > No, because if LLVM_IAS=1 then the assembler support need not be checked. > > Also, if your version of GCC supports DWARF Version 5, but your > version of GAS does not, then I'm more inclined to not allow > CONFIG_DEBUG_INFO_DWARF5 to be selectable, rather than mix and match > or partially support this for one but not the other. Either all tools > used support DWARF 5, or you don't get to use DWARF 5. > Why? Does this actually cause any problems? It seems like the options for gcc can actually be very straightforward: you just need a cc-option check, and then add -gdwarf-N to both CFLAGS and AFLAGS and you're done. Adding the -Wa flag is superfluous and carries the risk of interfering with what the compiler driver does. Just let the gcc driver handle the details. Clang/IAS is almost as straightforward, with the only additional edge case being that for assembler files, DWARF 2 doesn't work, so the CFLAGS is the same -gdwarf-N, but AFLAGS gets -gdwarf-N only if N > 2. The messy case is only Clang/IAS=0, which needs to check the support from the external assembler, and needs CFLAGS of -gdwarf-N and AFLAGS of -Wa,--gdwarf-N, because Clang doesn't pass that option on to an external assembler. This is why I was asking if the assembler support check can be restricted to CC_IS_CLANG: nothing but Clang/IAS=0 actually requires that check.
