Assembly Language for Beginners

3.16. TOUPPER() FUNCTION

10 movsx eax, BYTE PTR c$[rsp]
11 sub eax, 32
12 jmp SHORT $LN3@toupper
13 jmp SHORT $LN1@toupper ; compiler artefact
14 $LN2@toupper:
15 movzx eax, BYTE PTR c$[rsp] ; unnecessary casting
16 $LN1@toupper:
17 $LN3@toupper: ; compiler artefact
18 ret 0
19 toupper ENDP

It’s important to notice that the input byte is loaded into a 64-bit local stack slot at line 3.

All the remaining bits ([8..63]) are untouched, i.e., contain some random noise (you’ll see it in debugger).

All instructions operate only on byte-level, so it’s fine.

The lastMOVZXinstruction at line 15 takes the byte from the local stack slot and zero-extends it to aint

32-bit data type.

Non-optimizing GCC does mostly the same:

Listing 3.71: Non-optimizing GCC 4.9 (x64)

toupper:

push rbp

mov rbp, rsp

mov eax, edi

mov BYTE PTR [rbp-4], al

cmp BYTE PTR [rbp-4], 96

jle .L2

cmp BYTE PTR [rbp-4], 122

jg .L2

movzx eax, BYTE PTR [rbp-4]

sub eax, 32

jmp .L3

.L2:

movzx eax, BYTE PTR [rbp-4]

.L3:

pop rbp

ret

One comparison operation

Optimizing MSVC does a better job, it generates only one comparison operation:

Listing 3.72: Optimizing MSVC 2013 (x64)

toupper PROC

lea eax, DWORD PTR [rcx-97]

cmp al, 25

ja SHORT $LN2@toupper

movsx eax, cl

sub eax, 32

ret 0

$LN2@toupper:

movzx eax, cl

ret 0

toupper ENDP

It was explained earlier how to replace the two comparison operations with a single one:3.10.2 on

page 505.

We will now rewrite this in C/C++:

int tmp=c-97;

if (tmp>25)

return c;

else

return c-32;