AMDGPU: Fix an interaction between WQM and polygon stippling

Summary:
This fixes a rare bug in polygon stippling with non-monolithic pixel shaders.

The underlying problem is as follows: the prolog part contains the polygon
stippling sequence, i.e. a kill. The main part then enables WQM based on the
_reduced_ exec mask, effectively undoing most of the polygon stippling.

Since we cannot know whether polygon stippling will be used, the main part
of a non-monolithic shader must always return to exact mode to fix this
problem.

Reviewers: arsenm, tstellarAMD, mareko

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D23131

llvm-svn: 280589

2 files changed, 1 insertions, 7 deletions

diff --git a/llvm/lib/Target/AMDGPU/SIInstructions.td b/llvm/lib/Target/AMDGPU/SIInstructions.td
index 9eb7a503a7c..60a571d51bf 100644
--- a/llvm/lib/Target/AMDGPU/SIInstructions.td
+++ b/llvm/lib/Target/AMDGPU/SIInstructions.td
@@ -1301,6 +1301,7 @@ def SI_RETURN : SPseudoInstSI <
   let isReturn = 1;
   let hasSideEffects = 1;
   let hasNoSchedulingInfo = 1;
+  let DisableWQM = 1;
 }
 
 let Defs = [M0, EXEC],
diff --git a/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp b/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp
index c8bfc5aa460..03efd1f3326 100644
--- a/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp
+++ b/llvm/lib/Target/AMDGPU/SIWholeQuadMode.cpp
@@ -219,13 +219,6 @@ char SIWholeQuadMode::scanInstructions(MachineFunction &MF,
       markInstruction(MI, Flags, Worklist);
       GlobalFlags |= Flags;
     }
-
-    if (WQMOutputs && MBB.succ_empty()) {
-      // This is a prolog shader. Make sure we go back to exact mode at the end.
-      Blocks[&MBB].OutNeeds = StateExact;
-      Worklist.push_back(&MBB);
-      GlobalFlags |= StateExact;
-    }
   }
 
   return GlobalFlags;