__builtin_(add|mul)_overflow methods generate duplicate operations if both operands are const

__builtin_(add|mul)_overflow methods generate duplicate operations if both operands are const

[David Shane Holden]

I requested a bugzilla account a few days ago and haven't heard
back so I'm just going to report this here.

#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>

bool add(uint8_t *r, const uint8_t *a, const uint8_t *b) {
   return __builtin_add_overflow(*a, *b, r);
}

bool mul(uint8_t *r, const uint8_t *a, const uint8_t *b) {
   return __builtin_mul_overflow(*a, *b, r);
}

int main() {
   uint8_t x;

   /* 64 + 64 should not overflow */
   x = 64;
   if (add(&x, &x, &x))
     printf("false positive: x=%i\n", x);

   /* 4 * 4 should not overflow */
   x = 4;
   if (mul(&x, &x, &x))
     printf("false positive: x=%i\n", x);

   /* 128 + 128 should overflow */
   x = 128;
   if (!add(&x, &x, &x))
     printf("false negative: x=%i\n", x);

   /* 16 * 16 should overflow */
   x = 16;
   if (!mul(&x, &x, &x))
     printf("false negative: x=%i\n", x);

   return 0;
}

$ gcc -g3 -O1 -o test test.c; ./test
false positive: x=128
false positive: x=16
false negative: x=0
false negative: x=0

The generated assembly correctly adds a with b and stores in r but
then it does the operation again before testing for carry and
returning.  If r is also one of the operands then the erroneous
second operation will use the computed value of r in place of the
original operand leading to an incorrect overflow result.

0000000000400895 <add>:
;   return __builtin_add_overflow(*a, *b, r);
   400895: 0f b6 02                      movzbl  (%rdx), %eax
   400898: 02 06                         addb    (%rsi), %al
   40089a: 88 07                         movb    %al, (%rdi)
   40089c: 0f b6 02                      movzbl  (%rdx), %eax  <<
   40089f: 02 06                         addb    (%rsi), %al   <<
   4008a1: 0f 92 c0                      setb    %al
; }
   4008a4: c3                            retq

00000000004008a5 <mul>:
;   return __builtin_mul_overflow(*a, *b, r);
   4008a5: 0f b6 06                      movzbl  (%rsi), %eax
   4008a8: f6 22                         mulb    (%rdx)
   4008aa: 88 07                         movb    %al, (%rdi)
   4008ac: 0f b6 06                      movzbl  (%rsi), %eax  <<
   4008af: f6 22                         mulb    (%rdx)        <<
   4008b1: 0f 90 c0                      seto    %al
; }
   4008b4: c3                            retq

This only seems to be triggered when both a *and* b are const.
Removing const from either or both operands generates the correct
assembly.

0000000000400855 <add>:
;   return __builtin_add_overflow(*a, *b, r);
   400855: 0f b6 02                      movzbl  (%rdx), %eax
   400858: 02 06                         addb    (%rsi), %al
   40085a: 88 07                         movb    %al, (%rdi)
   40085c: 0f 92 c0                      setb    %al
; }
   40085f: c3                            retq

0000000000400860 <mul>:
;   return __builtin_mul_overflow(*a, *b, r);
   400860: 0f b6 06                      movzbl  (%rsi), %eax
   400863: f6 22                         mulb    (%rdx)
   400865: 88 07                         movb    %al, (%rdi)
   400867: 0f 90 c0                      seto    %al
; }
   40086a: c3                            retq

Tested on the following systems:

$ uname -srm; gcc12 --version
FreeBSD 13.1-RELEASE-p2 amd64
gcc12 (FreeBSD Ports Collection) 12.2.0

$ uname -smr; gcc --version
Linux 6.1.2-1-MANJARO-ARM aarch64
gcc (GCC) 12.1.0

$ uname -srm; gcc --version
MINGW64_NT-10.0-22000 3.4.5.x86_64 x86_64
gcc.exe (Rev10, Built by MSYS2 project) 12.2.0

Poking godbolt indicates this was likely introduced in 9.4.

Re: __builtin_(add|mul)_overflow methods generate duplicate operations if both operands are const

[Jonathan Wakely]

On Sun, 12 Feb 2023 at 20:46, David Shane Holden via Gcc
<gcc@gcc.gnu.org> wrote:
>
> I requested a bugzilla account a few days ago and haven't heard
> back so I'm just going to report this here.

I've created your account now, so please post this to bugzilla, thanks!