I'm analysing a piece of inefficient code, but some of it is so confusing?
Original code:
#include <string.h>
void lowwer(char *str) {
for (int i = 0; i < strlen(str); ++i) {
str[i] -= ('A' - 'a');
}
}
Assembly code (generated by clang 13 with -Og option):
lowwer:
pushq %r14 # use saved-registers
pushq %rbx
pushq %rax
# guard do while
cmpb $0, (%rdi) # compare &str with null (check if strlen(str) == 0)
je .LBB0_3
# loop initialization
movq %rdi, %r14 # %r14 = str
xorl %ebx, %ebx # clear %rbx (for more compact encoding)
.LBB0_2: # =>This Inner Loop Header: Depth=1
addb $32, (%r14,%rbx) # subtract -32 from str[i] ('A' - 'a' = -32)
addq $1, %rbx # ++i
movq %r14, %rdi # seems meaningless here?
callq strlen@PLT
cmpq %rbx, %rax # check i < strlen(str)
ja .LBB0_2
.LBB0_3: # end
addq $8, %rsp # ???
popq %rbx # free registers
popq %r14
retq
movq %r14, %rdi is doing? It seemed meangingless because %r14 holding the string pointer and the rdi is the same.addq $8, %rsp. Looks clueless.rdi is a caller-saved register and is hence trashed by the call to strlen. To preserve its contents, the compiler emitted code to move its contents to r14, copying it back once every iteration as an argument to strlen.
The addq $8, %rsp instruction releases stack space previously allocated by pushq %rax. This stack space was allocated to satisfy the stack alignment requirements imposed by the amd64 SysV ABI.
Refer to the amd64 SysV ABI supplement for the full calling convention and a list of caller/callee saved registers.