Unverified Commit 52e7c52d authored by Palmer Dabbelt's avatar Palmer Dabbelt
Browse files

RISC-V: Stop relying on GCC's register allocator's hueristics 



GCC allows users to hint to the register allocation that a variable should be
placed in a register by using a syntax along the lines of

    function(...) {
        register long in_REG __asm__("REG");
    }

We've abused this a bit throughout the RISC-V port to access fixed registers
directly as C variables.  In practice it's never going to blow up because GCC
isn't going to allocate these registers, but it's not a well defined syntax so
we really shouldn't be relying upon this.  Luckily there is a very similar but
well defined syntax that allows us to still access these registers directly as
C variables, which is to simply declare the register variables globally.  For
fixed variables this doesn't change the ABI.

LLVM disallows this ambiguous syntax, so this isn't just strictly a formatting
change.

Signed-off-by: default avatarPalmer Dabbelt <palmerdabbelt@google.com>
parent 064223b9

arch/riscv/include/asm/current.h

+3 −2
Original line number Diff line number Diff line
@@ -17,6 +17,8 @@ 


struct task_struct;



register struct task_struct *riscv_current_is_tp __asm__("tp");



/*

 * This only works because "struct thread_info" is at offset 0 from "struct

 * task_struct".  This constraint seems to be necessary on other architectures
@@ -26,8 +28,7 @@ struct task_struct; 
 */

static __always_inline struct task_struct *get_current(void)

{

	register struct task_struct *tp __asm__("tp");

	return tp;

	return riscv_current_is_tp;

}



#define current get_current()

arch/riscv/kernel/process.c

+3 −2
Original line number Diff line number Diff line
@@ -22,6 +22,8 @@ 
#include <asm/switch_to.h>

#include <asm/thread_info.h>



unsigned long gp_in_global __asm__("gp");



extern asmlinkage void ret_from_fork(void);

extern asmlinkage void ret_from_kernel_thread(void);
@@ -107,9 +109,8 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long usp, 
	/* p->thread holds context to be restored by __switch_to() */

	if (unlikely(p->flags & PF_KTHREAD)) {

		/* Kernel thread */

		const register unsigned long gp __asm__ ("gp");

		memset(childregs, 0, sizeof(struct pt_regs));

		childregs->gp = gp;

		childregs->gp = gp_in_global;

		/* Supervisor/Machine, irqs on: */

		childregs->status = SR_PP | SR_PIE;

arch/riscv/kernel/stacktrace.c

+4 −3
Original line number Diff line number Diff line
@@ -19,6 +19,8 @@ struct stackframe { 
	unsigned long ra;

};



register unsigned long sp_in_global __asm__("sp");



void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,

			     bool (*fn)(unsigned long, void *), void *arg)

{
@@ -29,7 +31,7 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs, 
		sp = user_stack_pointer(regs);

		pc = instruction_pointer(regs);

	} else if (task == NULL || task == current) {

		const register unsigned long current_sp __asm__ ("sp");

		const register unsigned long current_sp = sp_in_global;

		fp = (unsigned long)__builtin_frame_address(0);

		sp = current_sp;

		pc = (unsigned long)walk_stackframe;
@@ -73,8 +75,7 @@ static void notrace walk_stackframe(struct task_struct *task, 
		sp = user_stack_pointer(regs);

		pc = instruction_pointer(regs);

	} else if (task == NULL || task == current) {

		const register unsigned long current_sp __asm__ ("sp");

		sp = current_sp;

		sp = sp_in_global;

		pc = (unsigned long)walk_stackframe;

	} else {

		/* task blocked in __switch_to */

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