[cfe-dev] one bit error

[Alexey Sidorin via cfe-dev]

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:
> 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.
>
> -- 
> Ashish Kumar Gahlot
> III year, UG
> Govt. Engg. College, Ajmer, India
>
>
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