[llvm-dev] How to add optimizations to InstCombine correctly?
Sanjay Patel via llvm-dev
llvm-dev at lists.llvm.org
Tue Sep 19 06:58:13 PDT 2017
For the tests that are changing, you should see if those changes are
improvements, regressions, or neutral. This is unfortunately not always
obvious for x86 asm, so feel free to just post those diffs in an updated
version of the patch at D37896.
If the test files have auto-generated assertions (look for this string on
the first line of the test file: "NOTE: Assertions have been autogenerated
by utils/update_llc_test_checks.py"...
and both of these do as of: https://reviews.llvm.org/rL313631 ), then it's
easy to observe the diffs by re-running that script after your code patch
is applied:
$ /path/to/update_llc_test_checks.py --llc=/path/to/local/and/new/llc
lea-3.ll
$ svn diff lea-3.ll
On Tue, Sep 19, 2017 at 5:23 AM, Haidl, Michael via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I am currently improving the D37896 to include the suggestions from
> Chad. However, running the lit checks for the x86 backend I observe some
> changes in the generated MC, e.g.:
>
> llvm/test/CodeGen/X86/lea-3.ll:13:10: error: expected string not found
> in input
> ; CHECK: leal ([[A0]],[[A0]],2), %eax
> ^
> <stdin>:10:2: note: scanning from here
> orq %rdi, %rax
> ^
> <stdin>:10:2: note: with variable "A0" equal to "%rdi"
> orq %rdi, %rax
> ^
> <stdin>:10:2: note: with variable "A0" equal to "%rdi"
> orq %rdi, %rax
> ^
> <stdin>:23:2: note: possible intended match here
> leal (,%rdi,4), %eax
> ^
> or:
>
> llvm/test/CodeGen/X86/mul-constant-i16.ll:40:13: error: expected string
> not found in input
> ; X86-NEXT: movzwl {{[0-9]+}}(%esp), %eax
> ^
> <stdin>:35:2: note: scanning from here
> movzwl 4(%esp), %ecx
> ^
> llvm/test/CodeGen/X86/mul-constant-i16.ll:272:13: error: expected string
> not found in input
> ; X86-NEXT: movzwl {{[0-9]+}}(%esp), %eax
> ^
> <stdin>:212:2: note: scanning from here
> movzwl 4(%esp), %ecx
> ^
>
> What is the right way to fix this? Is it ok to modify the tests to match
> the new generated pattern?
>
> Cheers,
> Michael
>
>
> Am 16.09.2017 um 15:46 schrieb Simon Pilgrim:
> > This conversation has (partially) moved on to D37896 now, but if
> > possible I was hoping that we could perform this in DAGCombiner and
> > remove the various target specific combines that we still have.
> >
> > At least ARM/AARCH64 and X86 have cases that can hopefully be
> > generalised and removed, but there will probably be a few legality/perf
> > issues that will occur.
> >
> > Simon.
> >
> >> On 14 Sep 2017, at 06:23, Craig Topper <craig.topper at gmail.com
> >> <mailto:craig.topper at gmail.com>> wrote:
> >>
> >> Probably in visitMUL in DAGCombiner.cpp to be target independent. Or
> >> in LowerMUL in X86ISelLowering.cpp to be X86 specific.
> >>
> >> Adding Simon. Simon, which were you thinking?
> >>
> >> ~Craig
> >>
> >> On Wed, Sep 13, 2017 at 10:06 PM, Haidl, Michael
> >> <michael.haidl at uni-muenster.de <mailto:michael.haidl at uni-muenster.de>>
> >> wrote:
> >>
> >> Hi Craig,
> >>
> >> thanks for digging into this. So InstCombine is the wrong place for
> >> fixing PR34474. Can you give me a hint where such an optimization
> >> should
> >> go into CodeGen? I am not really familiar with stuff that happens
> >> after
> >> the MidLevel.
> >>
> >> Cheers,
> >> Michael
> >>
> >> Am 13.09.2017 um 19:21 schrieb Craig Topper:
> >> > And that is less instructions. So from InstCombine's perspective
> the
> >> > multiply is the correct answer. I think this transformation is
> better
> >> > left to codegen where we know whether multiply or shift is truly
> better.
> >> >
> >> > ~Craig
> >> >
> >> > On Wed, Sep 13, 2017 at 10:18 AM, Craig Topper <
> craig.topper at gmail.com <mailto:craig.topper at gmail.com>
> >> > <mailto:craig.topper at gmail.com <mailto:craig.topper at gmail.com>>>
> wrote:
> >> >
> >> > There is in fact a transform out there somewhere that
> reverses yours.
> >> >
> >> > define i64 @foo(i64 %a) {
> >> > %b = shl i64 %a, 5
> >> > %c = add i64 %b, %a
> >> > ret i64 %c
> >> > }
> >> >
> >> > becomes
> >> >
> >> > define i64 @foo(i64 %a) {
> >> >
> >> > %c = mul i64 %a, 33
> >> >
> >> > ret i64 %c
> >> >
> >> > }
> >> >
> >> >
> >> > ~Craig
> >> >
> >> > On Wed, Sep 13, 2017 at 10:11 AM, Craig Topper
> >> > <craig.topper at gmail.com <mailto:craig.topper at gmail.com>
> >> <mailto:craig.topper at gmail.com <mailto:craig.topper at gmail.com>>>
> >> wrote:
> >> >
> >> > Your code seems fine. InstCombine can infinite loop if
> some
> >> > other transform is reversing your transform. Can you send
> the
> >> > whole patch and a test case?
> >> >
> >> > ~Craig
> >> >
> >> > On Wed, Sep 13, 2017 at 10:01 AM, Haidl, Michael via
> llvm-dev
> >> > <llvm-dev at lists.llvm.org
> >> <mailto:llvm-dev at lists.llvm.org> <mailto:llvm-dev at lists.llvm.org
> >> <mailto:llvm-dev at lists.llvm.org>>> wrote:
> >> >
> >> > Hi,
> >> >
> >> > I am working on PR34474 and try to add a new
> >> optimization to
> >> > InstCombine. Like in other parts of the visitMul
> >> function I
> >> > add a Shl
> >> > through the IR builder and create a new BinaryOp
> which I
> >> > return from
> >> > visitMul. If I understand correctly the new BinaryOp
> >> > returned from
> >> > visitMul should replace the original Instruction in
> the
> >> > Worklist.
> >> > However, I end up in an infinite loop and the
> >> Instruction I
> >> > try to
> >> > replace gets scheduled again and again. What is
> >> wrong in my
> >> > code?
> >> >
> >> > // Replace X * (2^C+/-1) with (X << C) -/+ X
> >> > APInt Plus1 = *IVal + 1;
> >> > APInt Minus1 = *IVal - 1;
> >> > int isPow2 = Plus1.isPowerOf2() ? 1 :
> >> Minus1.isPowerOf2() ?
> >> > -1 : 0;
> >> >
> >> > if (isPow2) {
> >> > APInt &Pow2 = isPow2 > 0 ? Plus1 : Minus1;
> >> > Value *Shl = Builder.CreateShl(Op0,
> >> Pow2.logBase2());
> >> > return BinaryOperator::Create(isPow2 > 0 ?
> >> > BinaryOperator::Sub :
> >> > BinaryOperator::Add, Shl, Op0);
> >> > }
> >> >
> >> > Thanks,
> >> > Michael
> >>
> >
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