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Add a few README.txt items.

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llvm-svn: 84059
This commit is contained in:
Dan Gohman 2009-10-13 23:58:05 +00:00
parent 86970a6ea6
commit d845fbc307
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lib/CodeGen/README.txt
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@ -206,3 +206,32 @@ Stack coloring improvments:
not spill slots.
2. Reorder objects to fill in gaps between objects.
e.g. 4, 1, <gap>, 4, 1, 1, 1, <gap>, 4 => 4, 1, 1, 1, 1, 4, 4
//===---------------------------------------------------------------------===//
The scheduler should be able to sort nearby instructions by their address. For
example, in an expanded memset sequence it's not uncommon to see code like this:
movl $0, 4(%rdi)
movl $0, 8(%rdi)
movl $0, 12(%rdi)
movl $0, 0(%rdi)
Each of the stores is independent, and the scheduler is currently making an
arbitrary decision about the order.
//===---------------------------------------------------------------------===//
Another opportunitiy in this code is that the $0 could be moved to a register:
movl $0, 4(%rdi)
movl $0, 8(%rdi)
movl $0, 12(%rdi)
movl $0, 0(%rdi)
This would save substantial code size, especially for longer sequences like
this. It would be easy to have a rule telling isel to avoid matching MOV32mi
if the immediate has more than some fixed number of uses. It's more involved
to teach the register allocator how to do late folding to recover from
excessive register pressure.