[PATCH] R600/SI: Re-initialize the m0 register after using it for indirect addressing

[Tom Stellard]

We need to store a value greater than or equal to the number of LDS
bytes allocated by the shader in the m0 register in order for LDS
instructions to work correctly.

We always initialize m0 at the beginning of a shader, but this register
is also used for indirect addressing offsets, so we need to
re-initialize it any time we use indirect addressing.
---
 lib/Target/R600/SILowerControlFlow.cpp | 86 +++++++++++++++++++---------------
 1 file changed, 49 insertions(+), 37 deletions(-)

diff --git a/lib/Target/R600/SILowerControlFlow.cpp b/lib/Target/R600/SILowerControlFlow.cpp
index 6601f2a..09030a2 100644
--- a/lib/Target/R600/SILowerControlFlow.cpp
+++ b/lib/Target/R600/SILowerControlFlow.cpp
@@ -86,6 +86,7 @@ private:
   void Kill(MachineInstr &MI);
   void Branch(MachineInstr &MI);
 
+  void InitM0ForLDS(MachineBasicBlock::iterator MI);
   void LoadM0(MachineInstr &MI, MachineInstr *MovRel);
   void IndirectSrc(MachineInstr &MI);
   void IndirectDst(MachineInstr &MI);
@@ -320,6 +321,14 @@ void SILowerControlFlowPass::Kill(MachineInstr &MI) {
   MI.eraseFromParent();
 }
 
+/// The m0 register stores the maximum allowable address for LDS reads and
+/// writes.  Its value must be at least the size in bytes of LDS allocated by
+/// the shader.  For simplicity, we set it to the maximum possible value.
+void SILowerControlFlowPass::InitM0ForLDS(MachineBasicBlock::iterator MI) {
+    BuildMI(*MI->getParent(), MI, MI->getDebugLoc(),  TII->get(AMDGPU::S_MOV_B32),
+            AMDGPU::M0).addImm(0xffffffff);
+}
+
 void SILowerControlFlowPass::LoadM0(MachineInstr &MI, MachineInstr *MovRel) {
 
   MachineBasicBlock &MBB = *MI.getParent();
@@ -333,52 +342,56 @@ void SILowerControlFlowPass::LoadM0(MachineInstr &MI, MachineInstr *MovRel) {
     BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B32), AMDGPU::M0)
             .addReg(Idx);
     MBB.insert(I, MovRel);
-    MI.eraseFromParent();
-    return;
-  }
+  } else {
 
-  assert(AMDGPU::SReg_64RegClass.contains(Save));
-  assert(AMDGPU::VReg_32RegClass.contains(Idx));
+    assert(AMDGPU::SReg_64RegClass.contains(Save));
+    assert(AMDGPU::VReg_32RegClass.contains(Idx));
 
-  // Save the EXEC mask
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B64), Save)
-          .addReg(AMDGPU::EXEC);
+    // Save the EXEC mask
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B64), Save)
+            .addReg(AMDGPU::EXEC);
 
-  // Read the next variant into VCC (lower 32 bits) <- also loop target
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::V_READFIRSTLANE_B32),
-          AMDGPU::VCC_LO)
-          .addReg(Idx);
+    // Read the next variant into VCC (lower 32 bits) <- also loop target
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::V_READFIRSTLANE_B32),
+            AMDGPU::VCC_LO)
+            .addReg(Idx);
 
-  // Move index from VCC into M0
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B32), AMDGPU::M0)
-          .addReg(AMDGPU::VCC_LO);
+    // Move index from VCC into M0
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B32), AMDGPU::M0)
+            .addReg(AMDGPU::VCC_LO);
 
-  // Compare the just read M0 value to all possible Idx values
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::V_CMP_EQ_U32_e32), AMDGPU::VCC)
-          .addReg(AMDGPU::M0)
-          .addReg(Idx);
+    // Compare the just read M0 value to all possible Idx values
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::V_CMP_EQ_U32_e32), AMDGPU::VCC)
+            .addReg(AMDGPU::M0)
+            .addReg(Idx);
 
-  // Update EXEC, save the original EXEC value to VCC
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_AND_SAVEEXEC_B64), AMDGPU::VCC)
-          .addReg(AMDGPU::VCC);
+    // Update EXEC, save the original EXEC value to VCC
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_AND_SAVEEXEC_B64), AMDGPU::VCC)
+            .addReg(AMDGPU::VCC);
 
-  // Do the actual move
-  MBB.insert(I, MovRel);
+    // Do the actual move
+    MBB.insert(I, MovRel);
 
-  // Update EXEC, switch all done bits to 0 and all todo bits to 1
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_XOR_B64), AMDGPU::EXEC)
-          .addReg(AMDGPU::EXEC)
-          .addReg(AMDGPU::VCC);
+    // Update EXEC, switch all done bits to 0 and all todo bits to 1
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_XOR_B64), AMDGPU::EXEC)
+            .addReg(AMDGPU::EXEC)
+            .addReg(AMDGPU::VCC);
 
-  // Loop back to V_READFIRSTLANE_B32 if there are still variants to cover
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_CBRANCH_EXECNZ))
-          .addImm(-7)
-          .addReg(AMDGPU::EXEC);
+    // Loop back to V_READFIRSTLANE_B32 if there are still variants to cover
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_CBRANCH_EXECNZ))
+            .addImm(-7)
+            .addReg(AMDGPU::EXEC);
 
-  // Restore EXEC
-  BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B64), AMDGPU::EXEC)
-          .addReg(Save);
+    // Restore EXEC
+    BuildMI(MBB, &MI, DL, TII->get(AMDGPU::S_MOV_B64), AMDGPU::EXEC)
+            .addReg(Save);
 
+  }
+  // FIXME: Are there any values other than the LDS address clamp that need to
+  // be stored in the m0 register and may be live for more than a few
+  // instructions?  If so, we should save the m0 register at the beginning
+  // of this function and restore it here.
+  InitM0ForLDS(&MI);
   MI.eraseFromParent();
 }
 
@@ -523,8 +536,7 @@ bool SILowerControlFlowPass::runOnMachineFunction(MachineFunction &MF) {
     MachineBasicBlock &MBB = MF.front();
     // Initialize M0 to a value that won't cause LDS access to be discarded
     // due to offset clamping
-    BuildMI(MBB, MBB.getFirstNonPHI(), DebugLoc(), TII->get(AMDGPU::S_MOV_B32),
-            AMDGPU::M0).addImm(0xffffffff);
+    InitM0ForLDS(MBB.getFirstNonPHI());
   }
 
   if (NeedWQM && MFI->ShaderType == ShaderType::PIXEL) {
-- 
1.8.1.5