Why is less than expression converted to less than or equal to the expression in gcc

Question

I am working on code optimization and go through the gcc internal components. I wrote a simple expression in my program, and I checked the gimple representation of this expression, and I was stuck why gcc did this. Say I have an expression:

if(i < 9) 

then in gimple view it will be converted to

 if(i <= 8) 

I do not know why to do this. Is it some kind of optimization, if so, can someone tell me how it can optimize our program?

Answer 1

Canonicalization helps detect CommonSubExpressions, for example:

 #include <stdio.h> int main(void) { unsigned u, pos; char buff[40]; for (u=pos=0; u < 10; u++) { buff[pos++] = (u <5) ? 'A' + u : 'a' + u; buff[pos++] = (u <=4) ? '0' + u : 'A' + u; } buff[pos++] = 0; printf("=%s=\n", buff); return 0; } 

GCC -O1 compiles this into:

  ... movl $1, %edx movl $65, %ecx .L4: cmpl $4, %eax ja .L2 movb %cl, (%rsi) leal 48(%rax), %r8d jmp .L3 .L2: leal 97(%rax), %edi movb %dil, (%rsi) movl %ecx, %r8d .L3: mov %edx, %edi movb %r8b, (%rsp,%rdi) addl $1, %eax addl $1, %ecx addl $2, %edx addq $2, %rsi cmpl $10, %eax jne .L4 movb $0, 20(%rsp) movq %rsp, %rdx movl $.LC0, %esi movl $1, %edi movl $0, %eax call __printf_chk ... 

GCC -O2 will actually delete the entire cycle and replace it with the destination stream.

Answer 2

Consider the following C code:

 int i = 10; if(i < 9) { puts("1234"); } 

And also the equivalent C code:

 int i = 10; if(i <= 8) { puts("asdf"); } 

Without optimization, both generate exactly the same build sequence:

 40052c: c7 45 fc 0a 00 00 00 movl $0xa,-0x4(%rbp) 400533: 83 7d fc 08 cmpl $0x8,-0x4(%rbp) 400537: 7f 0a jg 400543 <main+0x1f> 400539: bf 3c 06 40 00 mov $0x40063c,%edi 40053e: e8 d5 fe ff ff callq 400418 < puts@plt > 400543: .. .. .. .. .. .. .. 

Since I am not familiar with the implementation of GCC, I can only guess why the conversion is performed at all. Perhaps this facilitates the work of the code generator, since it should only handle one case. I expect someone to come up with a more definitive answer.