[tree-ssa] Kills by VDEFs in mark_def_sites

[tree-ssa] Kills by VDEFs in mark_def_sites

[Zdenek Dvorak]

Hello,

a comment in tree-into-ssa.c:mark_def_sites states that

/* Note that virtual definitions are irrelevant for computing KILLS
   because a VDEF does not constitute a killing definition of the
   variable.  */

and indeed, we do not set a bit in "kills" bitmap for VDEFs.

There are two things that make this part of code a bit weird:

1) For operand of VDEF and for VUSE we test whether the bit is set
   in the "kills" bitmap; however this is clearly useless, as there
   is no way how there could be anything set in this bitmap for virtual
   operands.

2) The comment above is moreorless useless as well.  Even if we set the
   bit in the "kills" bitmap, everything would work exactly the same
   way, since VDEF has an operand that is processed before the VDEF's
   result, so the basic block processed would be marked in the livein
   bitmap as well.

Are these observations correct, or am I missing something?

Zdenek

Re: [tree-ssa] Kills by VDEFs in mark_def_sites

[law]

In message <20040414213604.GA32480@atrey.karlin.mff.cuni.cz>, Zdenek Dvorak wri
tes:
 >Hello,
 >
 >a comment in tree-into-ssa.c:mark_def_sites states that
 >
 >/* Note that virtual definitions are irrelevant for computing KILLS
 >   because a VDEF does not constitute a killing definition of the
 >   variable.  */
 >
 >and indeed, we do not set a bit in "kills" bitmap for VDEFs.
 >
 >There are two things that make this part of code a bit weird:
 >
 >1) For operand of VDEF and for VUSE we test whether the bit is set
 >   in the "kills" bitmap; however this is clearly useless, as there
 >   is no way how there could be anything set in this bitmap for virtual
 >   operands.
 >
 >2) The comment above is moreorless useless as well.  Even if we set the
 >   bit in the "kills" bitmap, everything would work exactly the same
 >   way, since VDEF has an operand that is processed before the VDEF's
 >   result, so the basic block processed would be marked in the livein
 >   bitmap as well.
 >
 >Are these observations correct, or am I missing something?
I believe your observations are correct.  I believe this code only made
sense in the world where we still mixed real and virtual operands in
strange and not so wonderful ways.

I'll give things a spin with the kills bitmap gunk removed for virtuals.

jeff