GCC emits mov instead of lea

[SomePerson]

For my IDT I load function pointers into the IDT, but instead of loading the address of the function pointers gcc emits a mov and loads 8 bytes from the function instead. Additionally this works correctly if the function is in the same file.

Example:

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IDT_set_gate(0, ISR0, ...)

Emits:

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mov -0x...(%rip), %rsi

Instead of:

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lea -0x...(%rip), %rsi

But

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IDT_set_gate(0, IDT_initialize, ...)

With IDT_initialize in the same file emits the correct lea.

What is causing this and how can I fix this?

[nullplan]

There's not enough code here to tell what is going on. But I suspect you declare ISR0 with the wrong type in C. Typically, it should be something like

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extern const char ISR0[];

And I suspect you have something like

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extern unsigned long ISR0;

How close am I?

[SomePerson]

ISR0 is a function declared like this:

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void ISR0();

[iansjack]

Do you declare ISR0() in the file containing the IDT functions? If not the compiler will assume that ISR0 an int rather than a pointer to a function.

[SomePerson]

Yes and the same problem also occurs with functions that I can call that are in another file.

[iansjack]

I think, as null plan says, that we need to see the code.

[SomePerson]

isr.c:

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void ISR0();
void ISR1();
void ISR2();
void ISR3();
void ISR4();
...

void ISR_initialize()
{
    IDT_set_gate(0, ISR_initialize, GDT_KCS, 0, 0xf); // "Works"
    //	-> lea -0x1e(%rip), %rsi
    IDT_set_gate(0, ISR0, GDT_KCS, 0, 0xf); // Does not work
    //	-> mov -0x33a(%rip), %rsi
    IDT_set_gate(1, ISR1, GDT_KCS, 0, 0xf);
    IDT_set_gate(2, ISR2, GDT_KCS, 0, 0xf);
    IDT_set_gate(3, ISR3, GDT_KCS, 0, 0xf);
    IDT_set_gate(4, ISR4, GDT_KCS, 0, 0xf);
    ...
}

idt.c:

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void IDT_set_gate(int interrupt, void (*base)(), uint16_t segment, int privilege, int type)
{
    if(interrupt > 255)return;

    idt[interrupt].base_low = ((uintptr_t)base) & 0xffff;
    idt[interrupt].base_mid = (((uintptr_t)base) >> 16) & 0xffff;
    idt[interrupt].base_high = ((uintptr_t)base) >> 32;
    idt[interrupt].segment = segment;
    idt[interrupt].flags = (privilege << 13) | ((type & 0xf) << 8);
    idt[interrupt].flags |= IDT_ENABLE_FLAG;
}

[sebihepp]

Try &ISR0 instead of ISR0

[SomePerson]

1: I tried &ISR0, but it did not work. (Also that should be equivalent to ISR0 anyway)

2: I got around the problem by doing this:

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void (*isrs[])() = {
    ISR0, ISR1, ISR2, ISR3,
    ISR4, ISR5, ISR6, ISR7,
    ISR8, ISR9, ISR10, ISR11,
    ISR12, ISR13, ISR14, ISR15,
    ISR16, ISR17, ISR18, ISR19,
    ISR20, ISR21, ISR22, ISR23,
    ISR24, ISR25, ISR26, ISR27,
    ISR28, ISR29, ISR30, ISR31,
    ISR32, ISR33, ISR34, ISR35,
    ISR36, ISR37, ISR38, ISR39,
    ISR40, ISR41, ISR42, ISR43,
    ISR44, ISR45, ISR46, ISR47,
};

and then calling IDT_set_gate like this:

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IDT_set_gate(0, isrs[0], GDT_KCS, 0, 0xf);

[Octocontrabass]

How are you compiling and linking this code? That looks a bit like a PIC relocation that didn't get linked correctly.

[SomePerson]

Compiling:
gcc -Wall -Wextra -O3 -ffreestanding -nostdlib -c $< -o $@
Linking:
gcc -Tx86_64.ld -nostdlib $^ -lgcc -o $@
x86_64.ld:

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KERNEL_PM = 0x100000
KERNEL_VM = 0xffff800000100000
KERNEL_VM_OFFSET = KERNEL_VM - KERNEL_PM

ENTRY(k32_entry)
OUTPUT_FORMAT("binary")
SECTIONS
{
	. = KERNEL_PM;
	... k32 stuff
	. = ALIGN(4096);
	. += KERNEL_VM_OFFSET;
	.text : AT(ADDR(.text) - KERNEL_VM_OFFSET) {...}
	+ other sections
	+ discards
}

Also I am having the same problem again with an array (loads first 8 bytes instead of pointer).

[Octocontrabass]

gcc -Wall -Wextra -O3 -ffreestanding -nostdlib -c $< -o $@

You should use a cross-compiler. Since you're not using a cross-compiler, you get whatever default options the package maintainers decided to use when they built your copy of GCC, and I'd guess they decided to enable PIC by default. I'm not sure why your resulting binary isn't statically linked correctly, but disabling PIC (-fno-pic) will fix that. If you disable PIC you'll also need to specify the code model according to the address where your kernel is linked. The current address you're using requires the large code model (-mcmodel=large), but if you change it to 0xFFFFFFFF80000000 or higher, you can use the kernel code model (-mcmodel=kernel).

You also need to disable the red zone (-mno-red-zone), and you probably want to disable the use of extended registers (-mgeneral-regs-only).

[SomePerson]

Thank you.