Example of optimization (clang & gcc)

sum.c

int sum(int n) {
    int acc = 0;
    for (int i = 0; i <= n; i++) {
        acc += i;
    }
    return acc;
}

sum_clang_O0.ll

; ModuleID = '2.c'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.11.0"

; Function Attrs: nounwind ssp uwtable
define i32 @sum(i32 %n) #0 {
entry:
  %n.addr = alloca i32, align 4
  %acc = alloca i32, align 4
  %i = alloca i32, align 4
  store i32 %n, i32* %n.addr, align 4
  store i32 0, i32* %acc, align 4
  store i32 0, i32* %i, align 4
  br label %for.cond

for.cond:                                         ; preds = %for.inc, %entry
  %0 = load i32, i32* %i, align 4
  %1 = load i32, i32* %n.addr, align 4
  %cmp = icmp sle i32 %0, %1
  br i1 %cmp, label %for.body, label %for.end

for.body:                                         ; preds = %for.cond
  %2 = load i32, i32* %i, align 4
  %3 = load i32, i32* %acc, align 4
  %add = add nsw i32 %3, %2
  store i32 %add, i32* %acc, align 4
  br label %for.inc

for.inc:                                          ; preds = %for.body
  %4 = load i32, i32* %i, align 4
  %inc = add nsw i32 %4, 1
  store i32 %inc, i32* %i, align 4
  br label %for.cond

for.end:                                          ; preds = %for.cond
  %5 = load i32, i32* %acc, align 4
  ret i32 %5
}

attributes #0 = { nounwind ssp uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"clang version 3.8.0 (tags/RELEASE_380/final)"}

sum_clang_O0.s

	.section	__TEXT,__text,regular,pure_instructions
	.macosx_version_min 10, 11
	.intel_syntax noprefix
	.globl	_sum
	.align	4, 0x90
_sum:                                   ## @sum
	.cfi_startproc
## BB#0:                                ## %entry
	push	rbp
Ltmp0:
	.cfi_def_cfa_offset 16
Ltmp1:
	.cfi_offset rbp, -16
	mov	rbp, rsp
Ltmp2:
	.cfi_def_cfa_register rbp
	mov	dword ptr [rbp - 4], edi
	mov	dword ptr [rbp - 8], 0
	mov	dword ptr [rbp - 12], 0
LBB0_1:                                 ## %for.cond
                                        ## =>This Inner Loop Header: Depth=1
	mov	eax, dword ptr [rbp - 12]
	cmp	eax, dword ptr [rbp - 4]
	jg	LBB0_4
## BB#2:                                ## %for.body
                                        ##   in Loop: Header=BB0_1 Depth=1
	mov	eax, dword ptr [rbp - 12]
	add	eax, dword ptr [rbp - 8]
	mov	dword ptr [rbp - 8], eax
## BB#3:                                ## %for.inc
                                        ##   in Loop: Header=BB0_1 Depth=1
	mov	eax, dword ptr [rbp - 12]
	add	eax, 1
	mov	dword ptr [rbp - 12], eax
	jmp	LBB0_1
LBB0_4:                                 ## %for.end
	mov	eax, dword ptr [rbp - 8]
	pop	rbp
	ret
	.cfi_endproc


.subsections_via_symbols

sum_clang_O3.ll

; ModuleID = '2.c'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.11.0"

; Function Attrs: norecurse nounwind readnone ssp uwtable
define i32 @sum(i32 %n) #0 {
entry:
  %cmp6 = icmp slt i32 %n, 0
  br i1 %cmp6, label %for.cond.cleanup, label %for.body.preheader

for.body.preheader:                               ; preds = %entry
  %0 = zext i32 %n to i33
  %1 = add i32 %n, -1
  %2 = zext i32 %1 to i33
  %3 = mul i33 %0, %2
  %4 = lshr i33 %3, 1
  %5 = trunc i33 %4 to i32
  %6 = add i32 %5, %n
  br label %for.cond.cleanup

for.cond.cleanup:                                 ; preds = %for.body.preheader, %entry
  %acc.0.lcssa = phi i32 [ 0, %entry ], [ %6, %for.body.preheader ]
  ret i32 %acc.0.lcssa
}

attributes #0 = { norecurse nounwind readnone ssp uwtable "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="core2" "target-features"="+cx16,+fxsr,+mmx,+sse,+sse2,+sse3,+ssse3" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.module.flags = !{!0}
!llvm.ident = !{!1}

!0 = !{i32 1, !"PIC Level", i32 2}
!1 = !{!"clang version 3.8.0 (tags/RELEASE_380/final)"}

sum_clang_O3.s

	.section	__TEXT,__text,regular,pure_instructions
	.macosx_version_min 10, 11
	.intel_syntax noprefix
	.globl	_sum
	.align	4, 0x90
_sum:                                   ## @sum
	.cfi_startproc
## BB#0:                                ## %entry
	push	rbp
Ltmp0:
	.cfi_def_cfa_offset 16
Ltmp1:
	.cfi_offset rbp, -16
	mov	rbp, rsp
Ltmp2:
	.cfi_def_cfa_register rbp
	xor	eax, eax
	test	edi, edi
	js	LBB0_2
## BB#1:                                ## %for.body.preheader
	mov	ecx, edi
	lea	eax, [rdi - 1]
	imul	rax, rcx
	shr	rax
	add	eax, edi
LBB0_2:                                 ## %for.cond.cleanup
	pop	rbp
	ret
	.cfi_endproc


.subsections_via_symbols

sum_gcc_O0.s

	.section	__TEXT,__text,regular,pure_instructions
	.macosx_version_min 10, 11
	.globl	_sum
	.align	4, 0x90
_sum:                                   ## @sum
	.cfi_startproc
## BB#0:
	pushq	%rbp
Ltmp0:
	.cfi_def_cfa_offset 16
Ltmp1:
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
Ltmp2:
	.cfi_def_cfa_register %rbp
	movl	%edi, -4(%rbp)
	movl	$0, -8(%rbp)
	movl	$0, -12(%rbp)
LBB0_1:                                 ## =>This Inner Loop Header: Depth=1
	movl	-12(%rbp), %eax
	cmpl	-4(%rbp), %eax
	jg	LBB0_4
## BB#2:                                ##   in Loop: Header=BB0_1 Depth=1
	movl	-12(%rbp), %eax
	addl	-8(%rbp), %eax
	movl	%eax, -8(%rbp)
## BB#3:                                ##   in Loop: Header=BB0_1 Depth=1
	movl	-12(%rbp), %eax
	addl	$1, %eax
	movl	%eax, -12(%rbp)
	jmp	LBB0_1
LBB0_4:
	movl	-8(%rbp), %eax
	popq	%rbp
	retq
	.cfi_endproc


.subsections_via_symbols

sum_gcc_O3.s

	.section	__TEXT,__text,regular,pure_instructions
	.macosx_version_min 10, 11
	.globl	_sum
	.align	4, 0x90
_sum:                                   ## @sum
	.cfi_startproc
## BB#0:
	pushq	%rbp
Ltmp0:
	.cfi_def_cfa_offset 16
Ltmp1:
	.cfi_offset %rbp, -16
	movq	%rsp, %rbp
Ltmp2:
	.cfi_def_cfa_register %rbp
	xorl	%eax, %eax
	testl	%edi, %edi
	js	LBB0_2
## BB#1:                                ## %.lr.ph.preheader
	movl	%edi, %ecx
	leal	-1(%rdi), %eax
	imulq	%rcx, %rax
	shrq	%rax
	addl	%edi, %eax
LBB0_2:                                 ## %._crit_edge
	popq	%rbp
	retq
	.cfi_endproc


.subsections_via_symbols