Saturated integer addition in C
I'm writing a C application that contains considerable amount of
saturated integer math. Beeing curious I took a glance on the
assembler gcc-4.3.1 generates for x86.
The problem seems to be that I haven't found any way of coding
that would result in overflow/carry dependent jumps or MMX/SSE2
(MOVD -> PADDD -> MOVD).
Any hints/ideas are welcome on how to write protable C code
for saturated integer math. (Inline assembler isn't an option.)
Below the C code, generated assembler and hand coded assembler
for saturated addition of two 32 bit signed integers.
The best C for gcc I've come up with so far:
| #include <limits.h>
|
| int sum_saturated_gcc(int a, int b){
| int r = a + b;
|
| if(0 <= r)
| if((a < 0) && (b < 0))
| r = INT_MIN;
| else if ((0 < a) && (0 < b))
| r = INT_MAX;
| return r;
| }
The generated assembler (-m32 -O3 -mmmx -msse -msse2 -msse3):
| .file "a.c"
| .text
| .p2align 4,,15
| .globl sum_saturated_gcc
| .type sum_saturated_gcc, @function
| sum_saturated_gcc:
| pushl %ebp
| movl %esp, %ebp
| movl 12(%ebp), %edx
| addl 8(%ebp), %edx
| jns .L6
| .L2:
| movl %edx, %eax
| popl %ebp
| ret
| .p2align 4,,7
| .p2align 3
| .L6:
| movl 8(%ebp), %eax
| testl %eax, %eax
| js .L7
| .L3:
| movl 8(%ebp), %ecx
| testl %ecx, %ecx
| jle .L2
| movl 12(%ebp), %eax
| testl %eax, %eax
| movl $2147483647, %eax
| cmovg %eax, %edx
| jmp .L2
| .p2align 4,,7
| .p2align 3
| .L7:
| movl 12(%ebp), %eax
| testl %eax, %eax
| jns .L3
| movl $-2147483648, %edx
| jmp .L2
| .size sum_saturated_gcc, .-sum_saturated_gcc
| .ident "GCC: (Gentoo 4.3.1-r1 p1.1) 4.3.1"
| .section .note.GNU-stack,"",@progbits
Hand coded assembler: plain x86, no MMX/SSE2 (MOVD -> PADDD -> MOVD)
| .text
| .globl sum_saturated
| .type sum_saturated, @function
| sum_saturated:
| # prolog
| pushl %ebp
| movl %esp, %ebp
|
| # add
| movl 8(%ebp), %eax
| addl 12(%ebp), %eax
|
| # check overflow
| jno .L_done
|
| # check type of overflow
| js .L_max
|
| .L_min:
| # overflow to positive
| movl $-2147483648, %eax
| jmp .L_done
|
| .L_max:
| # overflow to negative
| movl $2147483647, %eax
|
| .L_done:
| # epilog
| popl %ebp
| ret
| .size sum_saturated, .-sum_saturated
