On sh-elf, the function foo in 20010422-1.c

unsigned int foo(unsigned int x)
{
  if (x < 5)
    x = 4;
  else
    x = 8;
  return x;
}

is compiled to

foo:
        mov.l   r14,@-r15
        mov     r15,r14
        mov     #4,r1
        cmp/hi  r1,r1
        bf/s    .L6
        mov     #4,r0
        mov     #8,r0
.L6:
        mov     r14,r15
        mov.l   @r15+,r14
        rts
        nop

with -O1.  Thus the first parameter register r4 is ignored.
For mips-elf, foo is assembled to

foo:
        .frame  $sp,0,$31               # vars= 0, regs= 0/0, args= 0, gp= 0
        .mask   0x00000000,0
        .fmask  0x00000000,0
        .set    noreorder
        .set    nomacro

        sltu    $2,$2,5
        beq     $2,$0,$L2
        nop
	...

with -O1 where $4 should be register for x.
It started after the patch

r119711 | amacleod | 2006-12-11 06:25:40 +0900 (Mon, 11 Dec 2006) | 103 lines

and the first tree dump which differs before and after this patch
is .099t.optimized:

[r119710 .099t.optimized]
;; Function foo (foo)

Analyzing Edge Insertions.
foo (x)
{
  unsigned int x.24;

<bb 2>:
  if (x <= 4) goto <L5>; else goto <L4>;

<L5>:;
  x.24 = 4;
  goto <bb 4> (<L2>);

<L4>:;
  x.24 = 8;

<L2>:;
  return x.24;

}

[r119711 .099t.optimized]
;; Function foo (foo)

Analyzing Edge Insertions.
foo (x)
{
  unsigned int x.24;

<bb 2>:
  if (x.24 <= 4) goto <L5>; else goto <L4>;

<L5>:;
  x = 4;
  goto <bb 4> (<L2>);

<L4>:;
  x = 8;

<L2>:;
  return x;

}