Compare two data size Assembly Analysis

Compare the size of two numbers, it is easy to write in C Language

int compare1(int x, int y){    if (x > y)    {        return 1;    }     else if (x == y)     {        return 0;    }    else    {        return -1;    }}

Use GCC to generate PowerPC PPU Assembly commands and use the default-O Option

The generated assembly code:

.compare1:.LFB39:stdu 1,-80(1).LCFI3:std 31,72(1).LCFI4:mr 31,1.LCFI5:mr 0,3mr 9,4stw 0,128(31)stw 9,136(31)lwz 9,128(31)lwz 0,136(31)cmpw 7,9,0ble 7,.L4li 0,1stw 0,48(31)b .L6.L4:lwz 0,128(31)lwz 9,136(31)cmpw 7,0,9bne 7,.L7li 9,0stw 9,48(31)b .L6.L7:li 0,-1stw 0,48(31).L6:lwz 9,48(31)extsw 0,9mr 3,0ld 11,0(1)ld 31,-8(11)mr 1,11blr

Of course, many of these commands are not clear, but some write operations have some pressure on the stack. Below we will use the GCC-O3 option for optimization.

.compare1:.LFB39:cmpw 7,3,4li 3,1ble 7,.L11.L6:extsw 3,3blr.L11:li 3,0beq 7,.L6li 3,-1b .L6

The code is much less, but I use the if statement to redirect a lot. We all know that the jump is time-consuming.

So I wrote another method:

int compare2(int x, int y){    return x < y ? -1 : x == y ? 0 : 1;}

The above code is very concise. Let's take a look at the Code Compiled by GCC-O3. The default-O Option Code will not be listed. You can try the following by yourself:

. Compare2 :. lfb40: XOR, 4 // R3 = x, r4 = ycmpw, 3, 4 srawi 11, 0, 31 // R0 arithmetic right shift 31-bit Li 9,-1xor, 0 subf 3, 3, 11 // This command means R3 = R11-r3blt 7 ,. l15srwi 9, 3, 31 // R9 = R3 logic shifts 31 bits to the right. l15: extsw 3, 9blr

I probably know how to compile, so I wrote one myself based on the compilation practice:

<Span style = "white-space: pre"> </span> Li R9,-1 // R9 =-1 <span style = "white-space: pre "> </span> subf R6, R3, R4 <span style =" white-space: pre "> </span> cmpwi cr7, R6, 0 // Cal the diff <span style = "white-space: pre"> </span> BLT cr7 ,. out // jump to store-1 <span style = "white-space: pre"> </span> Li R7, 0 <span style = "white-space: pre "> </span> subf R6, R6, R7 // Cal 0-diff, if the diff = 0, then R6 = 0, diff> 0, r6 <0 <span style = "white-space: pre"> </span> srwi R9, R6, 31 // mov the sign bit to R9 ,. out: <span style = "white-space: pre"> </span> Mr R3, R9 // R3 is the register that stores the returned value <span style = "white-space: pre "> </span> BLR

The above is the analysis of C and Assembly comparing the two numbers, but there are many jumps when using the if statement.

Any errors. Please indicate the source of the post.

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