Loop optimization, and microbenchmarking

I read a blog post "Reducing the Performance Impact of Debug Code in C Libraries" that describes a way to handle overhead of debug code (compile two versions with/without debug code enabled, and load the right version dynamically). The blog post's approach is a reasonable way to solve the problem, but the example illustrating the overhead of debug code surprised me:

#include <stdio.h>
#include <stdlib.h>

#define N 100000000 // Adjust for your machine

int main(void)
{
    int debug_on = !!getenv("DEBUG");
    unsigned long sum = 0;

    for (int i = 0 ; i < N ; i++) {
        for (int j = 0 ; j < N ; j++) {
#ifdef WITH_DEBUG
            if (debug_on)
                printf("Debug print.\n");
#endif
            sum += i;
        }
    }
    printf("Sum: %lu\n", sum);

    return 0;
}

The blog post claims that the version compiled with WITH_DEBUG defined (introducing the overhead of "if (false)") is 50% slower than the one compiled without it — I had thought that both versions would have the same running time, so I tested the program compiled with "gcc -O3", and the difference was even bigger on my computer...

The reason I thought both versions would have identical performance is that GCC has an optimization -funswitch-loops (enabled at -O3) that optimizes loops having if-statements where the condition is unchanged within the loop, such as

for (int j = 0 ; j < N ; j++) {
    if (debug_on)
        printf("Debug print.\n");
    sum += i;
}

The optimization duplicates the loop, optimizes one as if the condition is true and the other as if it is false, and selects the correct version at runtime. That is, the loop above is transformed to

if (debug_on) {
    for (int j = 0 ; j < N ; j++) {
        printf("Debug print.\n");
        sum += i;
    }
} else {
    for (int j = 0 ; j < N ; j++) {
        sum += i;
    }
}

so the performance should be the same with/without WITH_DEBUG defined — both should end up running code resulting from the loop without the if-statement!

I looked some into the details of what is happening. Compiling without defining WITH_DEBUG makes GCC determine that the inner loop

for (int j = 0 ; j < N ; j++) {
    sum += i;
}

calculates

sum += (unsigned long)i * N;

This, in turn, makes the outer loop calculate the sum of an arithmetic sequence, which the compiler knows how to turn into a constant. The result is that the whole program is transformed to the equivalent of

int main(void)
{
    getenv("DEBUG");
    unsigned long sum = 996882102603448320ul;
    printf("Sum: %lu\n", sum);

    return 0;
}

Compiling with WITH_DEBUG defined determines that the inner loop does not change debug_on, and the loop is "unswitched". The sums are determined to be "i * N" as in the previous case, and the compiler sees that both branches do the same calculation that can be moved out and combined. The result is that the inner loop is transformed to

if (debug_on) {
    for (int j = 0 ; j < N ; j++)
        printf("Debug print.\n");
}
sum += (unsigned long)i * N;

The outer loop could now be unswitched but that is not happening, so the compiler continues by noticing that the sum can be directly calculated as in the previous case, and the resulting optimized program is equivalent to

int main(void)
{
    int debug_on = !!getenv("DEBUG");
    for (int i = 0 ; i < N ; i++)
        if (debug_on)
            for (int j = 0 ; j < N ; j++)
                printf("Debug print.\n");

    unsigned long sum = 996882102603448320ul;
    printf("Sum: %lu\n", sum);

    return 0;
}

The reason the program is not fully optimized is that the optimization passes are not run first on the inner loop as in my description above — they are (mostly) run over the whole program, one optimization pass at a time, and each pass requires the code to be simple enough for the optimizations to trigger. In this case, the outer loop would have been unswitched if sum had been optimized before the unswitching pass was run on the outer loop. Dealing with this is one of the "fun" parts of compiler development — you implement an amazing optimization pass that can do wonders, but the stupid real-world code need to be simplified by other passes (later in the chain) before your pass can do its magic. It is, of course, easy to fix this by adding additional optimization passes before (and in the middle of) your new pass, but the users will complain that the compiler is too slow and switch to a faster, less optimizing, compiler... So one important part of compiler development is to ensure that the pass order is good enough to handle most reasonable cases with a limited number of optimization passes.

This also means that simple micro-benchmarking may not give a true and fair view of how code is optimized in reality — I have seen many cases where micro-benchmarks in the compiler's test suite are optimized as expected, while the optimization is almost guaranteed not to trigger for real world code due to pass ordering issues. So your micro-benchmark may show that a code construct X is faster than code construct Y (or that compiler Z is better than compiler W), but the behavior in more complex real-world usage may be very different...

Tentative variable definitions, and -fno-common

A comment in a Reddit thread on my previous blog post claims that the code is optimized if the variables are declared in the same translation unit as the usage. That is, the claim is that

int a;
int b;

int foo(void)
{
    return &a != &b;
}

compiles to code returning a constant value. This is true for C++

_Z3foov:
    movl    $1, %eax
    ret

but compiling as C still generates

foo:
    movl    $a, %eax
    cmpq    $b, %rax
    setne   %al
    movzbl  %al, %eax
    ret

The reason is that an external declaration without an initializer is what the C standard calls a tentative definition. The tentative definition works in essentially the same way as if it were defined extern (but with the difference that the linker creates the variable if it is not defined in any translation unit), so GCC must assume that the linker may assign the same address for a and b in the same way as for normal extern-declared variables. It is possible to make the variables "real" definitions by initializing them with a value, so changing the declarations to

int a = 0;
int b = 0;

enables the compiler to optimize the function in C too.

One other way to make GCC optimize this is to specify the command-line option -fno-common that disables generation of "common symbols" which are used for emitting tentative definitions. This has the benefit of improving code generation for some CPU architectures, such as ARM. As an example, consider the function

int a;
int b;

int foo(void)
{
    return a + b;
}

The ARM architecture need the variables' addresses placed in registers in order to access the values, so the compiler will generate code similar to

foo:
    ldr     r2, .L2         /* Load address of a */
    ldr     r3, .L2+4       /* Load address of b */
    ldr     r0, [r2]        /* Load a */
    ldr     r3, [r3]        /* Load b */
    add     r0, r0, r3
    bx      lr
.L2:
    .word   a
    .word   b

The compiler could do a better job if it knew how the variables are laid out in memory, as it then only needs to load one address and use relative addressing for the other. But that is not possible when a and b are tentative definitions, as the linker may place them wherever it chooses. Using -fno-common will, however, place a and b in the data (or BSS) segment exactly as they are laid out by the compiler, and the code can be optimized based on knowledge that b is placed right after a in memory¹

foo:
    ldr     r2, .L2         /* Load address of a */
    ldr     r0, [r2]        /* Load a */
    ldr     r3, [r2, #4]    /* Load b */
    add     r0, r0, r3
    bx      lr
.L2:
    .word   a

---

1. A real ARM compiler would use an ldmia instruction in this case, which is even better. But I choose to use this naive code generation in the example in order to show the general principle, without discussing too many ARM-specific details.

"missing" optimizations — constant address comparison

Sometimes the compiler intentionally fails to optimize certain constructs for arguably good reasons. For example, compiling the following with GCC

extern int a;
extern int b;

int foo(void)
{
    return &a != &b;
}

generates code doing a comparison

foo:
    movl    $a, %eax
    cmpq    $b, %rax
    setne   %al
    movzbl  %al, %eax
    ret

even though the C standard ensures the addresses of a and b are different.

It seems to be a bit unclear why GCC keeps this comparison, but the discussion in the bug 78035 mentions the C defect report DR #078, and the expressiveness of the ELF format. DR #078 notes that

unsigned int h(void)
{
    return memcpy != memmove;
}

may return 0, which happens on implementations where the C standard library uses the same code for memcpy and memmove (the C language cannot do that, but the standard library does not need to be written in C). This does not mean that the compiler must be able to handle different symbols mapping to the same address — it only says that C programs must not assume too much about the standard library. But ELF supports exporting multiple symbols for the same address, and GCC tries to honor ELF possibilities (such as the symbol interposition that is limiting optimizations for shared libraries).

I'm not convinced it makes sense for GCC to keep these comparisons in the generated code — other optimizations, such as alias analysis, treats global symbols as having different addresses, so it is likely that other optimizations will make the code fail if it has two symbols with the same address. For example,

extern int a;
extern int b;

int foo(void)
{
    a = 1;
    b = 5;
    a++;
    return &a != &b;
}

optimizes the accesses to a and b as if they have different addresses, even though the comparison is emitted:

foo:
    movl    $a, %eax
    movl    $5, b(%rip)
    movl    $2, a(%rip)
    cmpq    $b, %rax
    setne   %al
    movzbl  %al, %eax
    ret

This missing optimization does probably not make any difference in reality (although I could imagine some macro or template that relies on this being optimized), but this inconsistency in what is optimized annoys me...

Inlining — shared libraries are special

The way shared libraries work affect how the code can be optimized, so GCC must be more conservative with inlining when building shared libraries (i.e. when compiling with -fpic or -fPIC).

Consider the functions

int foo(void)
{
    return 23;
}

int bar(void)
{
    return 19 + foo();
}

Compiling this with "gcc -O3" inlines foo into bar

foo:
    movl    $23, %eax
    ret
bar:
    movl    $42, %eax
    ret

but that is not the case when compiling using "gcc -O3 -fPIC"

foo:
    movl    $23, %eax
    ret
bar:
    subq    $8, %rsp
    call    foo@PLT
    addq    $8, %rsp
    addl    $19, %eax
    ret

The reason is that ELF permits symbols in shared libraries to be overridden by the dynamic linker — a typical use case is to use LD_PRELOAD to load a debug library that contains logging versions of some functions. This has the effect that GCC cannot know that it is the foo above that really is called by bar, and thus cannot inline it. It is only exported symbols that can be overridden, so anonymous namespaces and static functions are optimized as usual, as are functions defined as "extern inline" (the compiler is told to inline, so it may assume the function will not be overridden).

The missed optimizations from this are especially noticeable when doing link-time optimization — the benefit of LTO is that the compiler can see the whole library and inline between files, but this is not possible if those functions may be replaced. This problem makes all interprocedural optimizations (such as devirtualization) ineffective, not only inlining.

There are two ways to get GCC to optimize shared libraries in the same way as normal code

• Use the command line option -fno-semantic-interposition
• Avoid exporting unnecessary symbols by passing -fvisibility=hidden to the compiler, and manually export the needed symbols using a linker export map or by decorating the functions with

  __attribute__((__visibility__("default")))