[MachineCSE] Prevent CSE of non-local convergent instrs

At the moment, MachineCSE allows CSE-ing convergent instrs which are
non-local to each other. This can cause illegal codegen as convergent
instrs are control flow dependent. The patch prevents non-local CSE of
convergent instrs by adding a check in isProfitableToCSE and rejecting
CSE-ing if we're considering CSE-ing non-local convergent instrs. We
can still CSE convergent instrs which are in the same control flow
scope, so the patch purposely does not make all convergent instrs
non-CSE candidates in isCSECandidate.

https://reviews.llvm.org/D101187

diff --git a/llvm/lib/CodeGen/MachineCSE.cpp b/llvm/lib/CodeGen/MachineCSE.cpp
index 199fe2d..de90d0f 100644 (file)
--- a/llvm/lib/CodeGen/MachineCSE.cpp
+++ b/llvm/lib/CodeGen/MachineCSE.cpp
@@ -433,6 +433,11 @@ bool MachineCSE::isProfitableToCSE(Register CSReg, Register Reg,
                                    MachineBasicBlock *CSBB, MachineInstr *MI) {
   // FIXME: Heuristics that works around the lack the live range splitting.
 
+  MachineBasicBlock *BB = MI->getParent();
+  // Prevent CSE-ing non-local convergent instructions.
+  if (MI->isConvergent() && CSBB != BB)
+    return false;
+
   // If CSReg is used at all uses of Reg, CSE should not increase register
   // pressure of CSReg.
   bool MayIncreasePressure = true;
@@ -455,7 +460,6 @@ bool MachineCSE::isProfitableToCSE(Register CSReg, Register Reg,
   // an immediate predecessor. We don't want to increase register pressure and
   // end up causing other computation to be spilled.
   if (TII->isAsCheapAsAMove(*MI)) {
-    MachineBasicBlock *BB = MI->getParent();
     if (CSBB != BB && !CSBB->isSuccessor(BB))
       return false;
   }

diff --git a/llvm/test/CodeGen/AMDGPU/GlobalISel/no-cse-nonlocal-convergent-instrs.mir b/llvm/test/CodeGen/AMDGPU/GlobalISel/no-cse-nonlocal-convergent-instrs.mir
new file mode 100644 (file)
index 0000000..f41b759
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/GlobalISel/no-cse-nonlocal-convergent-instrs.mir
@@ -0,0 +1,73 @@
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx1010 -o - -run-pass=machine-cse %s | FileCheck %s
+
+# Check that we don't CSE non-local convergent instrs. Otherwise, reusing defs
+# of convergent instrs from different control flow scopes can cause illegal
+# codegen. Previously, the swizzle in bb2 would be CSE-ed in favor of using the
+# swizzle in bb1 despite bb2 being a different control flow scope.
+
+# CHECK-LABEL: name: no_cse
+# CHECK: bb.1.if.then
+# CHECK: [[SWIZZLE1:%[0-9]+]]:vgpr_32 = DS_SWIZZLE_B32 [[SRC:%[0-9]+]], 100, 0, implicit $exec
+# CHECK-NEXT: V_ADD_CO_U32_e64 [[SWIZZLE1]], {{%[0-9]+}}, 0, implicit $exec
+# CHECK-NEXT: S_CMP_LT_I32 {{.*}} implicit-def $scc
+# CHECK-NEXT: S_CBRANCH_SCC1 %bb.3, implicit $scc
+# CHECK-NEXT: S_BRANCH %bb.2
+# CHECK: bb.2.if.then.if.then
+# CHECK: [[SWIZZLE2:%[0-9]+]]:vgpr_32 = DS_SWIZZLE_B32 [[SRC]], 100, 0, implicit $exec
+# CHECK-NEXT: V_ADD_CO_U32_e64 [[SWIZZLE2]], {{%[0-9]+}}, 0, implicit $exec
+
+--- |
+  define amdgpu_kernel void @no_cse(i32 addrspace(1)*, i32, i1) {
+  entry:
+    unreachable
+  if.then:
+    unreachable
+  if.then.if.then:
+    unreachable
+  if.then.phi:
+    unreachable
+  exit:
+    unreachable
+  }
+...
+---
+name: no_cse
+tracksRegLiveness: true
+body:             |
+  bb.0.entry:
+    liveins: $sgpr4_sgpr5
+    %0:sgpr_64(p4) = COPY $sgpr4_sgpr5
+    %1:sreg_64_xexec = S_LOAD_DWORDX2_IMM %0(p4), 0, 0
+    %2:sreg_64_xexec = S_LOAD_DWORDX2_IMM %0(p4), 2, 0
+    %3:sreg_64 = COPY %1
+    %4:sreg_32 = COPY %2.sub1
+    %5:sreg_32 = S_MOV_B32 42
+    S_CMP_EQ_U32 %4, %5, implicit-def $scc
+    %6:vgpr_32 = COPY %5, implicit $exec
+    S_CBRANCH_SCC1 %bb.4, implicit $scc
+    S_BRANCH %bb.1
+  
+  bb.1.if.then:
+    %7:sreg_32 = COPY %2.sub0
+    %8:vgpr_32 = COPY %7
+    %9:vgpr_32 = DS_SWIZZLE_B32 %8, 100, 0, implicit $exec
+    %10:vgpr_32, %21:sreg_32 = V_ADD_CO_U32_e64 %9, %5, 0, implicit $exec
+    S_CMP_LT_I32 %7, %5, implicit-def $scc
+    S_CBRANCH_SCC1 %bb.3, implicit $scc
+    S_BRANCH %bb.2
+  
+  bb.2.if.then.if.then:
+    %11:sreg_32 = S_MOV_B32 64
+    %12:vgpr_32 = DS_SWIZZLE_B32 %8, 100, 0, implicit $exec
+    %13:vgpr_32, %24:sreg_32 = V_ADD_CO_U32_e64 %12, %11, 0, implicit $exec
+  
+  bb.3.if.then.phi:
+    %14:vgpr_32 = PHI %10, %bb.1, %13, %bb.2
+  
+  bb.4.exit:
+    %15:vgpr_32 = PHI %6, %bb.0, %14, %bb.3
+    %16:vreg_64 = COPY %3
+    FLAT_STORE_DWORD %16, %15, 0, 0, implicit $exec, implicit $flat_scr
+    S_ENDPGM 0
+
+...