[LLVMbugs] [Bug 3495] New: LocalSpiller:: RewriteMBB does not propagate the information about AvailableSpills between MBBs

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3495

           Summary: LocalSpiller::RewriteMBB does not propagate the
                    information about AvailableSpills between MBBs
           Product: libraries
           Version: trunk
          Platform: All
               URL: http://lists.cs.uiuc.edu/pipermail/llvmdev/2009-
                    February/019976.html
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Register Allocator
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: romixlev at yahoo.com
                CC: llvmbugs at cs.uiuc.edu


LocalSpiller::RewriteMBB seems not to propagate the information
about e.g. Spills between MBBs.In many cases, where MBB B1 has only
one predecessor MBB B2, B1 could reuse the information about the
physical registers that are in the live-out set of B2. This could help
to e.g. eliminate some useless reloads from spill slots, if the value
is available on the required physical register already. For example,
in the example below, the marked "movl    12(%esp), %ecx" instruction
could be eliminated.

.LBB2_2:        # bb31
        movl    12(%esp), %ecx
        movl    8(%esp), %eax
        cmpl    $0, up+28(%eax,%ecx,4)
        je      .LBB2_9 # bb569
.LBB2_3:        # bb41         ; <--- bb31 is the only predecessor of bb41
        movl    12(%esp), %ecx ; <--- This could be eliminated!!!
        movl    4(%esp), %eax
        cmpl    $0, down(%eax,%ecx,4)
        je      .LBB2_9 # bb569


It is also worth mentioning, that currently reloads from spill slots
are not recorded in the Spills set using the addAvailable method, as
far as I can see. Wouldn't it make sense?

I have the feeling that  these improvements are rather easy to achieve
and would not require too much changes to the LocalSpiller. Probably,
we just need to keep the live-out set of the MBB around after
rewriting it, so that its successors can use it in some cases as
initial value for the Spills set.

For more information and test-case files please see this mail-thread:

http://lists.cs.uiuc.edu/pipermail/llvmdev/2009-February/019976.html


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