Q39

[Anding]

How do we multiply rax by 9 using exactly one instruction?

Would this be acceptable as an alternative?

IMUL rax, 9

[sayon]

For a long time it was impossible to use IMUL with more than one operand. Now there is a form that to multiply a register in one command it:

IMUL r64, r/m64, imm32 (from Intel reference manual, volume 2)

So the alternative using imul is possible, but it will look like that:

imul rax, rax, 9

lea is still used by compilers to multiply to certain constants, because it is faster than imul

For example:

/* main.c */
int start( int c ) {
   return c * 9;
}

Compiling with optimizations on without linking with C standard lib (avoids cluttering the disassembly):

> gcc -O3 main.c -o main
> objdump -M intel-mnemonic -d main
main:     file format elf64-x86-64


Disassembly of section .text:

0000000000400150 <_start>:
  400150:	8d 04 ff             	lea    eax,[rdi+rdi*8]
  400153:	c3                   	ret    

A less optimized code can implement the same functionality by using e.g. shifts and adds, which is still faster than imul.