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// Copyright (c) 2021 Johannes Stoelp
#include <common.h>
#if !defined(__linux__) || !defined(__x86_64__)
# error "Only supported on linux(x86_64)!"
#endif
void* memset(void* s, int c, size_t n) {
asm volatile(
"cld"
"\n"
"rep stosb"
: "+D"(s), "+c"(n)
: "a"(c)
: "memory");
return s;
}
void* memcpy(void* d, const void* s, size_t n) {
// Cases to distinguish resulting from s and d pointers.
//
// Case 1 - same
// |------------|
// s s+n
// d d+n
//
// -> Nothing to copy.
//
// Case 2 - disjunct
// |------------| |------------|
// s s+n d d+n
//
// -> Nothing to worry, just copy the bytes from s to d.
//
// Case 3 - head overlap
// |------------|
// s s+n
// |------------|
// d d+n
//
// -> Destructive copy for s but all bytes get properly copied from s to d.
// The user gets what he/she asked for.
//
// Case 4 - tail overlap
// |------------|
// s s+n
// |------------|
// d d+n
//
// -> With a simple forward copy we would override the tail of s while
// copying into the head of d. This is destructive for s but we would
// also copy "wrong" bytes into d when we copy the tail of s (as it is
// already overwritten).
// This copy could be done properly by copying backwards (on x86 we
// could use the direction flag for string operations).
// -> We don't support this here as it is not needed any of the examples.
// Case 4.
ERROR_ON(s <= d && d < (void*)((unsigned char*)s + n), "memcpy: Unsupported overlap!");
// Case 1.
if (d == s) {
return d;
}
// Case 2/3.
asm volatile(
"cld"
"\n"
"rep movsb"
: "+D"(d), "+S"(s), "+c"(n)
:
: "memory");
return d;
}
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