RFR(S): 8136820 Generate better code for some Unsafe addressing patterns

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8136820/webrev.00/

The main loop in: 

    static int[] array; 
    static long base; 
    static int test1() { 
        int res = 0; 
        for (int i = 0; i < 100; i++) { 
            long address = (((long) i) << 2) + base; 
            res += UNSAFE.getInt(array, address); 
        } 
        return res; 
    } 

is compiled as: 

0b0 B2: # B2 B3 <- B1 B2 Loop: B2-B2 inner Freq: 101.492 
0b0 movslq R9, R10 # i2l 
0b3 salq R9, #2 
0b7 addq R9, [R11 + #112 (8-bit)] # long 
0bb addl RAX, [R8 + R9] # int 
0bf incl R10 # int 
0c2 cmpl R10, #100 
0c6 jl,s B2 # loop end P=0.990147 C=6732.000000 

but could be compiled as: 

0b2 B2: # B2 B3 <- B1 B2 Loop: B2-B2 inner Freq: 101.492 
0b2 addl RAX, [R8 + pos R10 << #2] # int 
0b6 incl R10 # int 
0b9 cmpl R10, #100 
0bd jl,s B2 # loop end P=0.990147 C=6732.000000 

base and array are loop invariant so array + base can be computed before the loop.

Roland.