[llvm-dev] Fwd: [PATCH] D20841: TII: Add documentation about conditional exits. NFC

Kyle Butt via llvm-dev llvm-dev at lists.llvm.org
Tue May 31 16:19:15 PDT 2016

I was hoping to get feedback on this proposal formatted as a patch. I'd
like to teach AnalyzeBranch to handle conditional returns on PPC, but the
currently documented API for AnalyzeBranch doesn't support it.

I propose the following enhancement to the API so that I can teach
AnalyzeBranch about conditional return instructions.
---------- Forwarded message ----------
From: Kyle Butt <kyle+llvm at iteratee.net>
Date: Tue, May 31, 2016 at 4:07 PM
Subject: [PATCH] D20841: TII: Add documentation about conditional exits. NFC
To: kyle+llvm at iteratee.net
Cc: llvm-commits at lists.llvm.org

iteratee created this revision.
iteratee added a subscriber: llvm-commits.
iteratee set the repository for this revision to rL LLVM.

Currently the documented API for AnalyzeBranch leaves no place for
conditional returns. This leaves a blindspot after shrinkwrapping on
platforms with conditional return instructions.

The existing API can be used to handle conditional returns by using a null
and a non-empty condition list. A null TBB should never mean fallthrough
with a
non-empty condition list, as that wouldn't make any sense. This leaves a
natural way to represent conditional return branches.

Next up is to add support to PPC for AnalyzeBranch and conditional




Index: include/llvm/Target/TargetInstrInfo.h
--- include/llvm/Target/TargetInstrInfo.h
+++ include/llvm/Target/TargetInstrInfo.h
@@ -458,6 +458,13 @@
   ///    'false' destination in FBB, and a list of operands that evaluate
   ///    condition.  These operands can be passed to other TargetInstrInfo
   ///    methods to create new branches.
+  /// 5. If this block ends with a conditional ret with fallthrough, it
+  ///    null for TBB and FBB and returns a non-empty list of operands that
+  ///    evaluate the condition. These operands can be used to create new
+  ///    branches.
+  /// 6. If the block ends with a conditional return followed by an
+  ///    unconditional branch, the 'false' destination is FBB, while TBB is
+  ///    null. Cond is non-empty as above.
   /// Note that RemoveBranch and InsertBranch must be implemented to
   /// cases where this method returns success.
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