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gcc 128-bits bug.

 
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pixel



Joined: 30 Jan 2004
Posts: 791
PostPosted: Thu Jan 13, 2005 8:06 pm    Post subject: gcc 128-bits bug. Reply with quote
Just to warn you that I've discovered a bug in the current toolchain when dealing with 128 bits data. Here is an example:

Code:
#include <tamtypes.h>

void foobar(int a, int b, int c, int d, int e, int f, int g, int h, u128 i);

void test() {
    foobar(1, 2, 3, 4, 5, 6, 7, 8, 9);
}


That will generate:
Code:
00000000 <test>:
   0:   27bdffd0        addiu   sp,sp,-48
   4:   ffbf0020        sd      ra,32(sp)
   8:   ffbe0010        sd      s8,16(sp)
   c:   03a0f02d        move    s8,sp
  10:   700014a9        por     v0,zero,zero
  14:   24020009        li      v0,9
  18:   7fa20000        sq      v0,0(sp)
  1c:   24040001        li      a0,1
  20:   24050002        li      a1,2
  24:   24060003        li      a2,3
  28:   24070004        li      a3,4
  2c:   24080005        li      t0,5
  30:   24090006        li      t1,6
  34:   240a0007        li      t2,7
  38:   0c000000        jal     0 <test>
                        38: R_MIPS_26   foobar
  3c:   240b0008        li      t3,8
  40:   03c0e82d        move    sp,s8
  44:   dfbf0020        ld      ra,32(sp)
  48:   dfbe0010        ld      s8,16(sp)
  4c:   03e00008        jr      ra
  50:   27bd0030        addiu   sp,sp,48


That is, the 9th argument is placed onstack (the sq v0, 0(sp)) and AFAIK, the stack is 64-bits aligned, not 128 bits, and gcc obviously doesn't do any work here to align the stack pointer before doing the sq. So this code have a 50% success rate. This does apply to varargs as well.
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pixel



Joined: 30 Jan 2004
Posts: 791
PostPosted: Thu Jan 13, 2005 9:45 pm    Post subject: Reply with quote
Okaaaay, after some talks on IRC, and some investigations, it seems that our nice gcc is doing 16-bytes alignments when setting up the stack, so that it knows where is the current arguments. Adding another argument to the previous code gives correct code. Sorry for the confusion ;)
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