https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108789
Bug ID: 108789
Summary: __builtin_(add|mul)_overflow methods generate
duplicate operations if both operands are const
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: dpejesh at yahoo dot com
Target Milestone: ---
#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 around godbolt indicates this was likely introduced in 9.4.
