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

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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: Palmer Dabbelt <palmerdabbelt@google.com>
This commit is contained in:
Palmer Dabbelt
2020-02-27 11:07:28 -08:00
parent 064223b947
commit 52e7c52d2d
3 changed files with 10 additions and 7 deletions
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arch/riscv/kernel/stacktrace.c
+4 -3
View File
@@ -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 */