EnableFastISel

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1

Hi,

In SelectionDAGISel::SelectAllBasicBlocks

  if (TM.Options.EnableFastISel)
    FastIS = TLI->createFastISel(*FuncInfo, LibInfo);

followed by

  if (!FastIS) {
    LowerArguments(Fn);
  } else {
  
The above implies that implementing FastIS is optional.

In contrast to that, testing whether FastIS is actually been used is
done by testing if TM.Options.EnableFastISel is set.

For example in SelectionDAGISel::LowerArguments

    SDB->setValue(&Arg, Res);
    if (!TM.Options.EnableFastISel && Res.getOpcode() ==
                                                   ISD::BUILD_PAIR) {
      if (LoadSDNode *LNode =
          dyn_cast<LoadSDNode>(Res.getOperand(0).getNode()))
        if (FrameIndexSDNode *FI =
            dyn_cast<FrameIndexSDNode>(LNode->getBasePtr().getNode()))
        FuncInfo->setArgumentFrameIndex(&Arg, FI->getIndex());
    }

Is it in BUG or am I missing something?

Thanks,
Yaniv

2

Not the right mailing list?

3

Not the right mailing list?

This is the right mailing list; sometimes people just lose track of an email.

Looking briefly at your question, yes, I agree it's a bug, but probably not one which has much impact, given the code it controls.

-Eli

4

Thanks.

The following are the output of

    long test(long a, long b, long c)
    {
        return a + b + c;
    }

where target triple is "sparc". The first is the output when
EnableFastISel is set. The second is an output when EnableFastISel
is cleard. As you can see it affects the output code.

        .text
        .file "test2.c"
        .globl test ! -- Begin function test
        .p2align 2
        .type test,@function
test: ! @test
! BB#0: ! %entry
        add %sp, -120, %sp
        mov %o2, %o3
        mov %o1, %o4
        mov %o0, %o5
        st %o0, [%sp+116]
        st %o1, [%sp+112]
        st %o2, [%sp+108]
        ld [%sp+116], %o0
        ld [%sp+112], %o1
        add %o0, %o1, %o0
        ld [%sp+108], %o1
        add %o0, %o1, %o0
        st %o3, [%sp+104] ! 4-byte Folded Spill
        st %o4, [%sp+100] ! 4-byte Folded Spill
        st %o5, [%sp+96] ! 4-byte Folded Spill
        retl
        add %sp, 120, %sp
.Lfunc_end0:
        .size test, .Lfunc_end0-test
                                        ! -- End function

        .ident "clang version 5.0.0 (tags/RELEASE_500/final)"
        .section ".note.GNU-stack"

        .text
        .file "test2.c"
        .globl test ! -- Begin function test
        .p2align 2
        .type test,@function
test: ! @test
! BB#0: ! %entry
        add %sp, -104, %sp
        st %o0, [%sp+100]
        st %o1, [%sp+96]
        st %o2, [%sp+92]
        ld [%sp+100], %o0
        ld [%sp+96], %o1
        add %o0, %o1, %o0
        ld [%sp+92], %o1
        add %o0, %o1, %o0
        retl
        add %sp, 104, %sp
.Lfunc_end0:
        .size test, .Lfunc_end0-test
                                        ! -- End function

        .ident "clang version 5.0.0 (tags/RELEASE_500/final)"
        .section ".note.GNU-stack"