https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109000
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-12 branch has been updated by Xi Ruoyao <xry...@gcc.gnu.org>: https://gcc.gnu.org/g:eb50091457a789e2f2a15c2e5d08e0d79d3195b1 commit r12-9225-geb50091457a789e2f2a15c2e5d08e0d79d3195b1 Author: Xi Ruoyao <xry...@xry111.site> Date: Thu Mar 2 18:05:23 2023 +0800 LoongArch: Stop -mfpu from silently breaking ABI [PR109000] In the toolchain convention, we describe -mfpu= as: "Selects the allowed set of basic floating-point instructions and registers. This option should not change the FP calling convention unless it's necessary." Though not explicitly stated, the rationale of this rule is to allow combinations like "-mabi=lp64s -mfpu=64". This will be useful for running applications with LP64S/F ABI on a double-float-capable LoongArch hardware and using a math library with LP64S/F ABI but native double float HW instructions, for a better performance. And now a case in Linux kernel has again proven the usefulness of this kind of combination. The AMDGPU DCN kernel driver needs to perform some floating-point operation, but the entire kernel uses LP64S ABI. So the translation units of the AMDGPU DCN driver need to be compiled with -mfpu=64 (the kernel lacks soft-FP routines in libgcc), but -mabi=lp64s (or you can't link it with the other part of the kernel). Unfortunately, currently GCC uses TARGET_{HARD,SOFT,DOUBLE}_FLOAT to determine the floating calling convention. This causes "-mfpu=64" silently allow using $fa* to pass parameters and return values EVEN IF -mabi=lp64s is used. To make things worse, the generated object file has SOFT-FLOAT set in the eflags field so the linker will happily link it with other LP64S ABI object files, but obviously this will lead to bad results at runtime. And for now all loongarch64 CPU models (-march settings) implies -mfpu=64 on by default, so the issue makes a single "-mabi=lp64s" option basically broken (fortunately most projects for eg the Linux kernel have used -msoft-float which implies both -mabi=lp64s and -mfpu=none as we've recommended in the toolchain convention doc). The fix is simple: use TARGET_*_FLOAT_ABI instead. I consider this a bug fix: the behavior difference from the toolchain convention doc is a bug, and generating object files with SOFT-FLOAT flag but parameters/return values passed through FPRs is definitely a bug. Bootstrapped and regtested on loongarch64-linux-gnu. Ok for trunk and release/gcc-12 branch? gcc/ChangeLog: PR target/109000 * config/loongarch/loongarch.h (FP_RETURN): Use TARGET_*_FLOAT_ABI instead of TARGET_*_FLOAT. (UNITS_PER_FP_ARG): Likewise. gcc/testsuite/ChangeLog: PR target/109000 * gcc.target/loongarch/flt-abi-isa-1.c: New test. * gcc.target/loongarch/flt-abi-isa-2.c: New test. * gcc.target/loongarch/flt-abi-isa-3.c: New test. * gcc.target/loongarch/flt-abi-isa-4.c: New test. (cherry picked from commit 75eccddef5784bc5e262af31f535267a9c4e993e)
