[LLVMdev] Poor register allocations vs gcc

[deco33000 at yandex.com]

Hello,

I have an issue with the llvm optimizations. I need to create object codes.

the -ON PURPOSE poor && useless- code :
---------------------------------------------------
#include <stdio.h>
#include <stdlib.h>

int ci(int a){

	return 23;

}
int flop(int a, char ** c){
	
	a += 71;

	int b = 0;

	if (a == 56){

		b = 69;
		b += ci(a);
	}

	puts("ok");
	return a + b;
}
--------------------------------------

Compiled that way (using the versions I downloaded and eventually compiled) :
clang_custom -std=c11 -O3 -march=native -c app2.c -S

against gcc:
gcc_custom -std=c11 -O3 -march=native -c app2.c -S

Versions (latest for each, downloaded just a few days ago):
gcc : 5.1
clang/llvm: clang+llvm-3.6.1-x86_64-apple-darwin

Host:
osx yosemite.

The assembly (cut to the essential):

LLVM:
	pushq	%rbp
	movq	%rsp, %rbp
	pushq	%r14
	pushq	%rbx
	movl	%edi, %r14d
	leal	71(%r14), %eax
	xorl	%ecx, %ecx
	cmpl	$56, %eax
	movl	$92, %ebx
	cmovnel	%ecx, %ebx
	leaq	L_.str(%rip), %rdi
	callq	_puts
	leal	71(%rbx,%r14), %eax
	popq	%rbx
	popq	%r14
	popq	%rbp
	retq

and the gcc one:

        pushq %rbp
	movl	$0, %eax
	movl	$92, %ebp
	pushq	%rbx
	leal	71(%rdi), %ebx
	leaq	LC1(%rip), %rdi
	subq	$8, %rsp
	cmpl	$56, %ebx
	cmovne	%eax, %ebp
	call	_puts
	addq	$8, %rsp
	leal	0(%rbp,%rbx), %eax
	popq	%rbx
	popq	%rbp
	ret

As we can see, llvm makes poor register allocations (ecx and r14), leading to more instructions for the same result.

Are there some optimizations I can bring on the table to avoid this ?
-- 
Jog