[PATCH] D150861: [AMDGPU][Uniformity] SI_IF and SI_ELSE pseudos are always divergent

[Sameer Sahasrabuddhe via Phabricator via llvm-commits]

sameerds created this revision.
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Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D150861

Files:
  llvm/include/llvm/ADT/GenericUniformityImpl.h
  llvm/lib/Target/AMDGPU/SIInstructions.td
  llvm/test/Analysis/UniformityAnalysis/AMDGPU/MIR/control-flow-intrinsics.mir


Index: llvm/test/Analysis/UniformityAnalysis/AMDGPU/MIR/control-flow-intrinsics.mir
===================================================================
--- /dev/null
+++ llvm/test/Analysis/UniformityAnalysis/AMDGPU/MIR/control-flow-intrinsics.mir
@@ -0,0 +1,39 @@
+# RUN: llc -mtriple=amdgcn-- -mcpu=gfx900 -run-pass=print-machine-uniformity -o - %s 2>&1 | FileCheck %s
+
+---
+name:            f1
+body:             |
+  ; CHECK-LABEL: MachineUniformityInfo for function: f1
+  bb.0:
+    successors: %bb.1, %bb.2
+
+    ; CHECK-NOT: DIVERGENT: %1
+    %1:sreg_64(s64) = G_IMPLICIT_DEF
+    ; CHECK: DIVERGENT: {{.*}} SI_IF
+    %2:sreg_64 = SI_IF %1, %bb.2, implicit-def $exec, implicit-def $scc, implicit $exec
+
+  bb.1:
+    SI_RETURN
+
+  bb.2:
+    G_BR %bb.1
+
+...
+
+---
+name:            f2
+body:             |
+  ; CHECK-LABEL: MachineUniformityInfo for function: f2
+  bb.0:
+    successors: %bb.1, %bb.2
+
+    ; CHECK-NOT: DIVERGENT: %1
+    %1:sreg_64(s64) = G_IMPLICIT_DEF
+    ; CHECK: DIVERGENT: {{.*}} SI_ELSE
+    %2:sreg_64 = SI_ELSE %1, %bb.2, implicit-def $exec, implicit-def $scc, implicit $exec
+
+  bb.1:
+    SI_RETURN
+
+  bb.2:
+    G_BR %bb.1
Index: llvm/lib/Target/AMDGPU/SIInstructions.td
===================================================================
--- llvm/lib/Target/AMDGPU/SIInstructions.td
+++ llvm/lib/Target/AMDGPU/SIInstructions.td
@@ -397,6 +397,7 @@
   let Constraints = "";
   let Size = 12;
   let hasSideEffects = 1;
+  let IsNeverUniform = 1;
 }
 
 def SI_ELSE : CFPseudoInstSI <
@@ -404,6 +405,7 @@
   (ins SReg_1:$src, brtarget:$target), [], 1, 1> {
   let Size = 12;
   let hasSideEffects = 1;
+  let IsNeverUniform = 1;
 }
 
 def SI_WATERFALL_LOOP : CFPseudoInstSI <
Index: llvm/include/llvm/ADT/GenericUniformityImpl.h
===================================================================
--- llvm/include/llvm/ADT/GenericUniformityImpl.h
+++ llvm/include/llvm/ADT/GenericUniformityImpl.h
@@ -1147,9 +1147,11 @@
 void GenericUniformityAnalysisImpl<ContextT>::print(raw_ostream &OS) const {
   bool haveDivergentArgs = false;
 
-  if (DivergentValues.empty()) {
-    assert(DivergentTermBlocks.empty());
-    assert(DivergentExitCycles.empty());
+  // Control flow instructions may be divergent even if their inputs are
+  // uniform. Thus, although exceedingly rare, it is possible to have a program
+  // with no divergent values but with divergent control structures.
+  if (DivergentValues.empty() && DivergentTermBlocks.empty() &&
+      DivergentExitCycles.empty()) {
     OS << "ALL VALUES UNIFORM\n";
     return;
   }


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