# [llvm-bugs] [Bug 33813] New: Optimization for removing same variable comparisons in loop: while(it != end1 && it != end2)

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Jul 17 01:17:30 PDT 2017

```https://bugs.llvm.org/show_bug.cgi?id=33813

Bug ID: 33813
Summary: Optimization for removing same variable comparisons in
loop: while(it != end1 && it != end2)
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedbugs at nondot.org
Reporter: antoshkka at gmail.com
CC: llvm-bugs at lists.llvm.org

Simple iteration by std::deque elements produces suboptimal code. For example

#include <deque>

unsigned sum(std::deque<unsigned> cont) {
unsigned sum = 0;
for (unsigned v : cont)
sum += v;

return sum;
}

produces the following loop:

.LBB0_1:                                # =>This Inner Loop Header: Depth=1
cmp     rsi, rcx
je      .LBB0_4
add     eax, dword ptr [rcx]
add     rcx, 4
cmp     rcx, rdx
jne     .LBB0_1
jmp     .LBB0_3

The loop has two comparisons in it and behaves close to the following C code:

unsigned sum_like_deque_does(unsigned** chunks, unsigned* end) {
unsigned sum = 0;

for (unsigned* it = *chunks; it != end; it = *(++chunks)) {
for (;it != end && it != *chunks + 128; ++it) {
sum += *it;
}
}

return sum;
}

Note the `it != end && it != *chunks + 128` condition. It could be simplified:
if `end` belongs to `[it, *chunks + 128]` change the condition to `it != end`
and use the condition `it != *chunks + 128` otherwise. Such optimization
removes the cmp from the loop and produces a much more faster loop:

.LBB2_3:                                #   Parent Loop BB2_2 Depth=1
add     eax, dword ptr [rcx]
add     rcx, 4
cmp     rdx, rcx
jne     .LBB2_3

Synthetic tests show up to 2 times better performance. Assembly outputs:
https://godbolt.org/g/vGs2qs

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