[llvm-dev] Particular type of loop optimization
Mehdi Amini via llvm-dev
llvm-dev at lists.llvm.org
Wed Feb 3 13:22:57 PST 2016
> On Feb 3, 2016, at 12:31 PM, Gleison Souza <gleison14051994 at gmail.com> wrote:
>
> Hi Mats,
>
> so, my overall goal is to insert annotations in the original C
> source. I produce these annotations after analyzing the bytecode that
> clangs gives me for that source. Thus, I need debugging information in
> the bytecode file. What are these annotations? They are comments
> relating variables which are pointers and their sizes, as program
> symbols.
This doesn’t sounds like a good solution to rely on debug information for that, as you may have figured out.
Have you looked at what Polly is doing do infer such bounds?
Some alternative more robust scheme would be to use pragmas instead of comment and have the front-end parse these pragma and lower the information you want in something like llvm.assume.
—
Mehdi
> For instance, if I have a program like this one below:
>
> void foo(int *v, int N) {
> int i;
> for (i = 0; i < N; i++) {
> v[i] = 0;
> }
> }
>
> I insert the following comment on this code:
>
> void foo(int *v, int N) {
> int i;
> // v is accessed within the loop, and its size is [0..N-1]
> for (i = 0; i < N; i++) {
> v[i] = 0;
> }
> }
>
> My prototype infers that 'v' is a pointer, and that it ranges on
> the region &v + 0 till &v + N - 1, where 'N' is a variable alive at
> the beginning of the loop. In the end, my analysis works for many
> kinds of programs, but I am having problems to infer that 'v' is an
> array with stable bounds whenever its base pointer (GetElementPtr)
> comes out of a load instruction which is inside the loop.
This example, as the one you sent just before (without global variable), does not have a load for the base address in the optimized IR.
—
Mehdi
> That's way I
> would like to be able to hoist out as many loads as I can. The
> question then is: what is the command line that I should pass to opt
> that is likely to hoist loads outside loops while preserving debugging
> info? Mehdi's suggestion got me really close (clang -O1 -S -o - -mllvm
> -print-after-all), but it is removing debugging information away from
> the code. Without debugging information, I can't associate names at
> the bytecode level with names at the C-source level.
>
> Regards,
>
> Gleison
>
> 2016-02-03 11:06 GMT-02:00 mats petersson <mats at planetcatfish.com <mailto:mats at planetcatfish.com>>:
> Just to be clear, "What are you actually trying to achieve?" was meant to ask what is your overall goal, and given that overall goal - not the transformation of the IR, but "why do you need to do this" - as a simple performance enhancement, or for some other reason?
>
> And like has been stated, alias-analysis will help here, but some cases, the compiler will not be able to certainly say that no access from the pointer will affect something else, meaning that the compiler NEEDS to take the conservative route and reload the pointer - it doesn't seem to be a -fstrict-alias flag for clang, like that of gcc.
>
> --
> Mats
>
> On 3 February 2016 at 12:05, Gleison Souza via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Thanks Mehdi,
>
> I tried to use this, but some debug information can be lost in these optimizations.
> I need write in the source file to insert information before the loops, and in
> some cases, I'm writing after the loop header.
>
> Please, take a look:
>
> int foo1 (int *a, int *b, int n) {
> int i, s= 0;
> for (i = 0; i < n; i++) {
> s = s * a[i];
> }
>
> for (i = 0; i < n; i++) {
> b[i] = a[i] + 3;
> s += a[i];
> }
> return s;
> }
>
> In this case, using the line obtained by this one in the LLVM's IR:
>
> Line = l->getStartLoc().getLine();
>
> The source file is cloned, and I'm writing my annotations inside the loop now.
> I can't do several modifications in the struct of code, just the necessary, thats the problem.
>
> Regards,
>
> Gleison
>
> 2016-02-03 1:07 GMT-02:00 Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>>:
>
>> On Feb 2, 2016, at 8:35 AM, Gleison Souza via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> Dear LLVMers,
>>
>> I am trying to implement a particular type of loop optimization, but I am having problems with global variables. To solve this problem, I would like to know if LLVM has some pass that moves loads outside loops. I will illustrate with an example. I want to transform this code below. I am writing in C for readability, but I am analysing LLVM IR:
>>
>> int *vectorE;
>>
>> void foo (int n) {
>> int i;
>> for (i = 0; i < n; i++)
>> vectorE[i] = i;
>> }
>>
>> into this one:
>>
>> int *vectorE;
>>
>> void foo (int n) {
>> int i;
>> int* aux = vectorE;
>> for (i = 0; i < n; i++)
>> aux[i] = i;
>> }
>
>
> Have you looked at the output of clang with optimization enabled (even O1)? For this C++ code the optimizer moves the access to the global in the loop preheader, and then the loop itself does not access the global at all, which seems to be what you’re looking for.
>
> Try: clang -O1 -S -o - -mllvm -print-after-all
>
> —
> Mehdi
>
>
>
>
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