one bit error

Clang Frontend
1

This is my first try on mailing list. Please correct if I am wrong.

On compiling this program using clang 3.9.1

#include <stdio.h>
#include <stdlib.h>

int main () {
int i,j;
static int x[4000][4000];
for (j = 0; j < 4000; j++) {
for (i = 0; i < 4000; i++) {
x[j][i] = i + j; }
}
}

got compiled to

mov dword ptr [rbp - 4], 0
mov dword ptr [rbp - 12], 0
.LBB0_1: # =>This Loop Header: Depth=1
cmp dword ptr [rbp - 12], 4000
jge .LBB0_8
mov dword ptr [rbp - 8], 0
.LBB0_3: # Parent Loop BB0_1 Depth=1
cmp dword ptr [rbp - 8], 4000

while gcc 7 gives this

mov DWORD PTR [rbp-8], 0
.L5:
cmp DWORD PTR [rbp-8], 3999
jg .L2
mov DWORD PTR [rbp-4], 0
.L4:
cmp DWORD PTR [rbp-4], 3999
jg .L3

I think it should be 3999.

2

What makes you think there is an issue? Do you observe an actual issue when you run your program?
(Did you notice that clang uses jge while gcc uses jg?)

3

If you take a look at the generated code, you will see that clang uses
         cmp dword ptr [rbp - 12], 4000
         jge .LBB0_8
which is equal (in C) to
   if (i >= 4000)

while gcc generates
         cmp DWORD PTR [rbp-8], 3999
         jg .L2
which is equal to
   if (i > 3999)

I don't see any difference here. Why do you think this is an issue?

21.01.2017 07:46, Ashish Gahlot via cfe-dev пишет:

4

sorry about that, I missed that detail. Thanks for correcting.