[RFH] - Less than optimal code compiling 252.eon -O2 for x86

[Fariborz Jahanian <fjahanian@apple.com>]

A source file mrSurfaceList.cc of 252.eon produces less efficient  
code initializing instance objects to 0 at -O2 than at -O1. Behavior  
is random and it does not happen on all x86  platforms and making the  
test smaller makes the problem go away. But here is what I found out  
is the cause.

When source is compiled with -O1 -march=pentium4,  'cse' phase sees  
the following pattern initializing a 'double' with 0.

(insn 18 13 19 0 (set (reg:SF 109)
         (mem/u/i:SF (symbol_ref/u:SI ("*LC11") [flags 0x2]) [0 S4  
A32])) -1 (nil)
     (nil))

(insn 19 18 20 0 (set (mem/s/j:DF (plus:SI (reg/f:SI 20 frame)
                 (const_int -32 [0xffffffffffffffe0])) [0  
objectBox.pmin.e+16 S8 A128])
         (float_extend:DF (reg:SF 109))) 86 {*extendsfdf2_sse} (nil)
     (nil))

Then fold_rtx routine  converts it into its reduced form, resulting  
in optimum code:

(insn 19 13 21 0 (set (mem/s/j:DF (plus:SI (reg/f:SI 20 frame)
                 (const_int -32 [0xffffffffffffffe0])) [0  
objectBox.pmin.e+16 S8 A128])
         (const_double:DF 0.0 [0x0.0p+0])) 64 {*movdf_nointeger} (nil)
     (nil))


But when the same source is compiled with -O2 march=pentium4, 'cse'  
phase sees a slightly different pattern (note that float_extend:DF  
has moved)

(insn 18 13 19 0 (set (reg:DF 109)
         (float_extend:DF (mem/u/i:SF (symbol_ref/u:SI ("*LC13")  
[flags 0x2]) [0 S4 A32]))) -1 (nil)
     (nil))

(insn 19 18 20 0 (set (mem/s/j:DF (plus:SI (reg/f:SI 20 frame)
                 (const_int -32 [0xffffffffffffffe0])) [0  
objectBox.pmin.e+16 S8 A128])
         (reg:DF 109)) 64 {*movdf_nointeger} (nil)
     (nil))

This cannot be simplified by fold_rtx, resulting in less efficient code.

Change in pattern is most likely because of additional tree  
optimization phases running at -O2. If so, then should the cse be  
taught to simplify the new rtl pattern. Or, the tree optimizer phase  
responsible for the less than optimal tree need be twiked to generate  
the same tree as with -O1?

Thanks, fariborz