sparse-llvm potential error in linearized output

sparse-llvm potential error in linearized output

[Dibyendu Majumdar]

I have the following test:

extern int printf(const char *, ...);
static char *randomletters = "agqwewbxklpfgytuorz";
int main(void)
{
 int i;
 for (i = 0; i < 20; i++) {
  int key;
  key = randomletters[i];
  printf("Inserting %d\n", key);
 }
 return 0;
}

The linearized output is:

main:
.L0:
        <entry-point>
        phisrc.32   %phi4(i) <- $0
        br          .L4
.L4:
        phi.32      %r1(i) <- %phi4(i), %phi5(i)
        setlt.32    %r2 <- %r1(i), $20
        br          %r2, .L1, .L5
.L1:
        load.64     %r3 <- 0[randomletters]
        scast.64    %r5 <- (32) %r1(i)
        add.64      %r6 <- %r3, %r5
        load.8      %r7 <- 0[%r6]
        scast.32    %r8 <- (8) %r7
        symaddr.64  %r9 <- <anon symbol:0000020914372028>
        call.32     %r11 <- printf, %r9, %r8
        add.32      %r13 <- %r1(i), $1
        phisrc.32   %phi5(i) <- %r13
        br          .L4
.L5:
        ret.32      $0

The following instruction looks incorrect to me:

        load.64     %r3 <- 0[randomletters]


Thanks and Regards

Dibyendu

Re: sparse-llvm potential error in linearized output

[Dibyendu Majumdar]

On 12 March 2017 at 16:27, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Mar 12, 2017 6:04 AM, "Dibyendu Majumdar" <mobile@majumdar.org.uk> wrote:
>         load.64     %r3 <- 0[randomletters]
>
>
> Looks fine to me.
>
> You have a variable called randomletters, and it's a pointer to a char. That
> 64-bit load just loads that pointer value from the variable into %r3.
>
> The sparse linearized format only has one memory access model: load with
> constant offset. The base can either be a symbol address (like in this case)
> or a register.
>

Ok thanks.

Regards
Dibyendu