Odd performance regression with -Os

[Andrew Haley <aph at redhat dot com>]

Here's a strange case of poor code generation with -Os.

unsigned short foo2 (unsigned char on_off, unsigned short *puls)
{
  return puls[on_off-1];
}

With -O2, it's fine:

	movzbl	%dil, %edi
	movzwl	-2(%rsi,%rdi,2), %eax
	ret

With -Os it's really weird:

	movzbl	%dil, %eax
	movl	$1, %edi
	subq	%rax, %rdi
	imulq	$-2, %rdi, %rdi
	movw	(%rdi,%rsi), %ax
	ret

The problem here seems to be that the FE (or the GIMPLE generation)
transforms

        addr + ((on_off-1)*2)

into

        addr + ((1-on_off)*-2)

and nothing in -Os has the wit to recover.  This badly affects targets
with no hardware multiplier, which end up calling libgcc to do the
multiply.

Is this a well-known problem?

Andrew.