commit 2c74fe977daf8d1d4c8b64cdd6d3a9999a4b4655
author Dan Gohman <dan433584@gmail.com> Wed Nov 08 21:59:51 2017 +0000
committer Dan Gohman <dan433584@gmail.com> Wed Nov 08 21:59:51 2017 +0000
tree 98b7ec31ec1bfdcc6869e3d884d44bc273bde172
parent c707c6f3a70f18ef8ceb1badea1de9965ce97e06

Add an @llvm.sideeffect intrinsic

This patch implements Chandler's idea [0] for supporting languages that
require support for infinite loops with side effects, such as Rust, providing
part of a solution to bug 965 [1].

Specifically, it adds an `llvm.sideeffect()` intrinsic, which has no actual
effect, but which appears to optimization passes to have obscure side effects,
such that they don't optimize away loops containing it. It also teaches
several optimization passes to ignore this intrinsic, so that it doesn't
significantly impact optimization in most cases.

As discussed on llvm-dev [2], this patch is the first of two major parts.
The second part, to change LLVM's semantics to have defined behavior
on infinite loops by default, with a function attribute for opting into
potential-undefined-behavior, will be implemented and posted for review in
a separate patch.

[0] http://lists.llvm.org/pipermail/llvm-dev/2015-July/088103.html
[1] https://bugs.llvm.org/show_bug.cgi?id=965
[2] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118632.html

Differential Revision: https://reviews.llvm.org/D38336

llvm-svn: 317729

llvm/lib/Analysis/AliasSetTracker.cpp

diff --git a/llvm/lib/Analysis/AliasSetTracker.cpp b/llvm/lib/Analysis/AliasSetTracker.cpp
index 025e811..b575944 100644
--- a/llvm/lib/Analysis/AliasSetTracker.cpp
+++ b/llvm/lib/Analysis/AliasSetTracker.cpp
@@ -436,6 +436,7 @@
       break;
       // FIXME: Add lifetime/invariant intrinsics (See: PR30807).
     case Intrinsic::assume:
+    case Intrinsic::sideeffect:
       return;
     }
   }

llvm/lib/Analysis/ValueTracking.cpp

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 2010858..9a0df74 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -433,6 +433,7 @@
       default: break;
       // FIXME: This list is repeated from NoTTI::getIntrinsicCost.
       case Intrinsic::assume:
+      case Intrinsic::sideeffect:
       case Intrinsic::dbg_declare:
       case Intrinsic::dbg_value:
       case Intrinsic::invariant_start:
@@ -3857,7 +3858,8 @@
     // FIXME: This isn't aggressive enough; a call which only writes to a global
     // is guaranteed to return.
     return CS.onlyReadsMemory() || CS.onlyAccessesArgMemory() ||
-           match(I, m_Intrinsic<Intrinsic::assume>());
+           match(I, m_Intrinsic<Intrinsic::assume>()) ||
+           match(I, m_Intrinsic<Intrinsic::sideeffect>());
   }
 
   // Other instructions return normally.

llvm/lib/Analysis/VectorUtils.cpp

diff --git a/llvm/lib/Analysis/VectorUtils.cpp b/llvm/lib/Analysis/VectorUtils.cpp
index 554d132..2becfbf 100644
--- a/llvm/lib/Analysis/VectorUtils.cpp
+++ b/llvm/lib/Analysis/VectorUtils.cpp
@@ -91,7 +91,8 @@
     return Intrinsic::not_intrinsic;
 
   if (isTriviallyVectorizable(ID) || ID == Intrinsic::lifetime_start ||
-      ID == Intrinsic::lifetime_end || ID == Intrinsic::assume)
+      ID == Intrinsic::lifetime_end || ID == Intrinsic::assume ||
+      ID == Intrinsic::sideeffect)
     return ID;
   return Intrinsic::not_intrinsic;
 }