[PATCH] D90361: Prevent LICM and machineLICM from hoisting convergent operations

[Xiaoqing Wu via Phabricator via llvm-commits]

xiaoqing_wu updated this revision to Diff 303195.
xiaoqing_wu added a comment.

Added a comment on why convergent instructions should not be hoisted or sunk in LICM and machineLICM.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D90361/new/

https://reviews.llvm.org/D90361

Files:
  llvm/lib/CodeGen/MachineLICM.cpp
  llvm/lib/Transforms/Scalar/LICM.cpp
  llvm/test/CodeGen/AMDGPU/machinelicm-convergent.mir
  llvm/test/Transforms/LICM/convergent.ll


Index: llvm/test/Transforms/LICM/convergent.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LICM/convergent.ll
@@ -0,0 +1,21 @@
+; RUN: opt < %s -S -licm | FileCheck %s
+
+; Check that we do not hoist convergent functions out of loop
+; CHECK: define i32 @test
+; CHECK: loop:
+; CHECK: call i32 @f
+
+define i32 @test(i32* nocapture noalias %x, i32* nocapture %y) {
+entry:
+  br label %loop
+
+loop:
+  %a = call i32 @f() nounwind readnone convergent
+  %exitcond = icmp ne i32 %a, 0
+  br i1 %exitcond, label %end, label %loop
+
+end:
+  ret i32 %a
+}
+
+declare i32 @f() nounwind readnone convergent
Index: llvm/test/CodeGen/AMDGPU/machinelicm-convergent.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AMDGPU/machinelicm-convergent.mir
@@ -0,0 +1,32 @@
+# RUN: llc -march=amdgcn  -run-pass=early-machinelicm -o - %s | FileCheck %s
+
+# Test to check machine LICM does not hoist convergent instructions,
+# DS_PERMUTE_B32 in this example.
+
+---
+# CHECK-LABEL: name: _amdgpu_cs_main
+# CHECK: bb.1:
+# CHECK: DS_PERMUTE_B32
+
+name: _amdgpu_cs_main
+body: |
+  bb.0:
+    liveins: $vgpr0, $vgpr1
+    successors: %bb.1
+
+    %0:vgpr_32 = COPY $vgpr0
+    %1:vgpr_32 = COPY $vgpr1
+
+  bb.1:
+    successors: %bb.1, %bb.2
+
+    %2:vgpr_32 = DS_PERMUTE_B32 %0, %1, 0, implicit $exec
+    %3:vgpr_32 = V_ADD_CO_U32_e32 %0, %2, implicit-def $vcc, implicit $exec
+    S_CBRANCH_SCC1 %bb.1, implicit undef $scc
+    S_BRANCH %bb.2
+
+  bb.2:
+    $vgpr0 = COPY %3
+    S_ENDPGM 0
+
+...
Index: llvm/lib/Transforms/Scalar/LICM.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LICM.cpp
+++ llvm/lib/Transforms/Scalar/LICM.cpp
@@ -1160,6 +1160,13 @@
     if (CI->mayThrow())
       return false;
 
+    // Convergent attribute has been used on operations that involve
+    // inter-thread communication which results are implicitly affected by the
+    // enclosing control flows. It is not safe to hoist or sink such operations
+    // across control flow.
+    if (CI->isConvergent())
+      return false;
+
     using namespace PatternMatch;
     if (match(CI, m_Intrinsic<Intrinsic::assume>()))
       // Assumes don't actually alias anything or throw
Index: llvm/lib/CodeGen/MachineLICM.cpp
===================================================================
--- llvm/lib/CodeGen/MachineLICM.cpp
+++ llvm/lib/CodeGen/MachineLICM.cpp
@@ -1043,6 +1043,13 @@
       !IsGuaranteedToExecute(I.getParent()))
     return false;
 
+  // Convergent attribute has been used on operations that involve inter-thread
+  // communication which results are implicitly affected by the enclosing
+  // control flows. It is not safe to hoist or sink such operations across
+  // control flow.
+  if (I.isConvergent())
+    return false;
+
   return true;
 }
 


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D90361.303195.patch
Type: text/x-patch
Size: 2921 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20201105/effd7d3f/attachment.bin>