On a i686 platform, the example below is miscompiled with -O1.
I expect this program to print -0.96.  Here's what it actually does:

$ g++ -O1 -o y y.cc
$ ./y
-1.288766
$ g++  -o y y.cc
$ ./y
-0.960000
$

The value that the optimized version prints is actually different
on each run of the program.

Here's the generated code for main(), 

.globl main
	.type	main, @function
main:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$24, %esp
	movl	%ebx, -8(%ebp)
	movl	%esi, -4(%ebp)
	andl	$-16, %esp
	subl	$16, %esp
	movl	$0, 12(%esp)
	movl	8(%esp), %ebx    ; this seems to load ebx with garbage???
	movl	12(%esp), %esi
	fldz
	fstl	8(%esp)
	fstpl	(%esp)
	call	_Z8x_from_zdd
	fstpl	4(%esp)
	movl	%ebx, 8(%esp)    ; this clobbers half of the arg with
                                 ; the garbage
	movl	%esi, 12(%esp)
	movl	$0, (%esp)
	call	_Z17local_to_trflocalidi
	movl	$0, %eax
	movl	-8(%ebp), %ebx
	movl	-4(%ebp), %esi
	movl	%ebp, %esp
	popl	%ebp
	ret

It looks to me like the error occurs during RTL generation.
Here's the tree dump from the t87.final_cleanup file (slightly trimmed):

void local_to_trflocal(int, double, int) (D.1741, x_loc, D.1743)
{
<bb 0>:
  printf (&"%f\n"[0], x_loc);
  return;
}

double x_from_z(double, double) (pitch, stereo)
{
<bb 0>:
  return -9.59999999999999964472863211994990706443786621094e-1 / cos (stereo);
}

int main() ()
{
<bb 0>:
  local_to_trflocal (0, x_from_z (0.0, 0.0), 0);
  return 0;
}


But in the 00.expand file, here's the sequence leading
to the local_to_trflocal() call:

(insn 27 26 28 1 (set (mem/i:DF (plus:SI (reg/f:SI 56 virtual-outgoing-args)
                (const_int 4 [0x4])) [0 S8 A32])
        (reg:DF 71)) -1 (nil)
    (nil))

(insn 28 27 29 1 (set (mem:DI (plus:SI (reg/f:SI 56 virtual-outgoing-args)
                (const_int 8 [0x8])) [0 S8 A8])
        (reg:DI 68)) -1 (nil)
    (nil))

(insn 29 28 30 1 (set (mem:SI (reg/f:SI 56 virtual-outgoing-args) [0 S4 A32])
        (const_int 0 [0x0])) -1 (nil)
    (nil))

(call_insn 30 29 31 1 (call (mem:QI (symbol_ref:SI ("_Z17local_to_trflocalidi") [flags 0x3] <function_decl 0xb7f0df80 local_to_trflocal>) [0 S1 A8])
        (const_int 16 [0x10])) -1 (nil)
    (nil)
    (nil))


Reg 71 here is the return value from x_from_z.
I don't know where the DI store to v-o-a+8 is coming from...

Environment:
System: Linux karma 2.6.12.1sss #2 Thu Jul 7 00:28:21 EDT 2005 i686 i686 i386 GNU/Linux
Architecture: i686

	<machine, os, target, libraries (multiple lines)>
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: /home/sss/gcc/gcc/configure --prefix=/usr/local/gcc --enable-threads=posix --enable-long-long --enable-languages=c,c++,f95

How-To-Repeat:

Compile with -O1

-----------------------------------------------------
//g++ -O1 -g -o y y.cc 

extern "C" double cos(double);
extern "C" int printf (...);


double x_from_z(double pitch=0, double stereo=0)
{
  return -0.96/cos(stereo);
}

void local_to_trflocal(int, double x_loc, int=0)
{
  printf ("%f\n", x_loc);
}


int main()
{
  local_to_trflocal(0, x_from_z());
  return 0;
}

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