benshi001 added a comment. In D123200#3495356 <https://reviews.llvm.org/D123200#3495356>, @aykevl wrote:
> @benshi001 I have been looking through the GCC code and I think avr-gcc also > has a special calling convention for many other functions, including > `__mulqi3` and `__mulhi3`. > > Source: > > 1. I think this is where the ABI is specified in the compiler: > https://github.com/gcc-mirror/gcc/blob/releases/gcc-5.4.0/gcc/config/avr/avr.md#L1543-L1549 > You can see that it multiplies R24 with R22 and stores the result in R24, > and clobbers R22 in the process. But no other registers. > 2. In this code sample <https://godbolt.org/z/cPE5ajrbv>, avr-gcc doesn't > save `char c` (`r20`) across the `__mulqi3` and `__mulhi3` calls, which is > normally call-clobbered. > > Therefore, I think we need to be a bit more careful with defining these AVR > builtins and check the ABI in avr-gcc first. > Also, we can make use of this and optimize the AVR backend more (you can see > that the Clang generated code is much worse than avr-gcc in the above > examples). Let us discuss that at https://github.com/llvm/llvm-project/issues/55279 Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D123200/new/ https://reviews.llvm.org/D123200 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
