https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119386
--- Comment #38 from Alexander Monakov <amonakov at gcc dot gnu.org> --- (In reply to Ard Biesheuvel from comment #37) > Yes, we can drop -mcmodel=kernel, and use -mcmodel=small instead. This is > why I'm not keen on relying on that - it is ill-defined and there is really > no need to have this special case. In the kernel, we are trying to move away > from all the special sauce in the toolchain - x86 especially is affected by > this, whereas arm64 and other architectures just use -mcmodel=small. The > primary sticking point is the relative cost of RIP-relative LEA vs 32-bit > absolute MOV but that gap appears to have been closing in recent designs. There's a couple places where GCC restricts offsets differently for -mcmodel=kernel vs. -mcmodel=small in the x86 backend. It's been determined it doesn't matter? How so? https://gcc.gnu.org/cgit/gcc/tree/gcc/config/i386/predicates.md#n239 > I'm not sure what that would solve. When linking the kernel, all > R_X86_64_PLT32 can be resolved directly, and so there is never the need for > a PLT in practice. The compiler does not have to care about this > distinction. Relaxing a CALL via a PLT into a direct one is much easier than > relaxing a GOT based data reference into a direct one. It's not just about the calls. On x86-64 it's less pronounced, but on arm64 telling the compiler up front that everything ends up in the final executable can improve codegen when referencing extern variables, for instance: //__attribute__((visibility("hidden"))) extern int a[]; int f(void) { return a[1]; } gcc -O2 -fpie gets you f: adrp x0, 0 <_GLOBAL_OFFSET_TABLE_> R_AARCH64_ADR_PREL_PG_HI21 _GLOBAL_OFFSET_TABLE_ ldr x0, [x0] R_AARCH64_LD64_GOTPAGE_LO15 a ldr w0, [x0, #4] ret and with the attribute uncommented, emulating what -fstatic-pie would do: f: adrp x0, 0 <a> R_AARCH64_ADR_PREL_PG_HI21 a+0x4 ldr w0, [x0] R_AARCH64_LDST32_ABS_LO12_NC a+0x4 ret
