<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Hi Shuxin,<div><br></div><div><div><div>On Aug 23, 2013, at 1:13 PM, Shuxin Yang <<a href="mailto:shuxin.llvm@gmail.com">shuxin.llvm@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
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Hi, Quentin:<br>
<br>
Shouldn't copy-propagation is supposed to already xform the code
in the way you want?<br></div></blockquote><div>No, not at the MI level.</div><div>Basically, the problem appears after instruction selection because of a combination of bit cast, vector extract and such.</div><div>Generally the MI IR is in good shape but in some (rare) cases, we end up with such things.</div><div><br></div><div>Thanks for the feedback.</div><div><br></div><div>-Quentin </div><br><blockquote type="cite"><div bgcolor="#FFFFFF" text="#000000">
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Shuxin <br>
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<div class="moz-cite-prefix">On 8/23/13 1:04 PM, Quentin Colombet
wrote:<br>
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<pre wrap="">Hi,
Here is a patch to rewrite copies to avoid cross register banks copies when possible.
Thanks for your reviews.
** Context **
By definition copies across register bank are not coalescable. Still, it may be possible to get rid of such a copy when the value is available in another register of the same register file.
Consider the following example, where capital and lower letters denote different register file:
b = copy A <-- cross-bank copy
…
C = copy b <-- cross-bank copy
This could have been optimized this way:
b = copy A <-- cross-bank copy
…
C = copy A <-- same-bank copy
Note: b and C's definitions may be in different basic blocks.
** Proposed Solution **
Add a peephole optimization that looks through a chain of copies leading to a cross-bank copy and reuses a source that is on the same register file if available.
This solution could also be used to get rid of some copies (e.g., A could have been used instead of C). However, we do not do so because:
- It may over constrain the coloring of the source register for coalescing.
- The register allocator may not be able to find a nice split point for the longer live-range, leading to more spill.
** Testcase?! **
This patch does not include a test case, because it is very difficult to reproduce this behavior with a reasonably small input.
Cheers,
-Quentin
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