https://llvm.org/bugs/show_bug.cgi?id=31347
Bug ID: 31347
Summary: AVR-GCC calling convention mismatch with stack
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: AVR
Assignee: unassignedb...@nondot.org
Reporter: dylanmcka...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
The calling convention for picking registers for arguments is described at
(https://gcc.gnu.org/wiki/avr-gcc).
I've found a mismatch between when AVR-GCC loads arguments from the stack, and
when we do.
AVR-GCC Test case:
avr-gcc -S tmp.c -o tmp.s -O2
#include <stdint.h>
typedef uint8_t i8;
typedef uint16_t i16;
typedef uint32_t i32;
typedef uint64_t i64;
void thing(i64 a, i64 b) {
*((i64*)0x4) = b;
}
This generates the following assembly to load argument 'b' from the stack
/* prologue: function */
/* frame size = 0 */
/* stack size = 8 */
.L__stack_usage = 8
ldi r30,lo8(4)
ldi r31,0
st Z,r10
std Z+1,r11
std Z+2,r12
std Z+3,r13
std Z+4,r14
std Z+5,r15
std Z+6,r16
std Z+7,r17
AVR-LLVM example:
define void @ret_void_args_i64_i64(i64 %a, i64 %b) {
store volatile i64 %b, i64* inttoptr (i64 4 to i64*)
ret void
}
This generates the following assembly:
sts 4, r18
sts 11, r17
sts 10, r16
sts 9, r15
sts 8, r14
sts 7, r13
sts 6, r12
sts 5, r11
sts 4, r10
ret
Clearly, we aren't following the calling convention in this case. We should be
loading 'b' from the stack.
This will break programs which take large numbers/big arguments on the stack.
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