[LLVMdev] Poor register allocations vs gcc

[Matthias Braun]

> On Jul 13, 2015, at 10:03 AM, deco33000 at yandex.com wrote:
> 
> 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 ?

As far as I know clang on OS X always sets up a frame pointer unless you explicitely use -fomit-frame-pointer. I think the reasoning being that dtrace and others rely on frame pointers being present.

I don't see why using %ecx would be a problem, there are no extra spill/reloads produced because of that.

- Matthias