[AMDGPU] Fix issues for backend divergence tracking

[dstuttard]

Summary

A change to use divergence analysis in the AMDGPU backend was getting formal
arguments incorrect (not tagged as divergent) unless they were VGPR0, VGPR1 or
VGPR2

For graphics shaders it is possible to have more than these passed in as VGPR

Modified the checking code to check for any VGPR registers passed in as formal
arguments.

Also, some intrinsics that are sources of divergence may have been lowered
during instruction selection and are missed on subsequent calls to
isSDNodeSourceOfDivergence - added the relevant AMDGPUISD checks as well.

Finally, the FunctionLoweringInfo tracks virtual registers that are live across
basic block boundaries. This is used to check for divergence of CopyFromRegister
registers using the DivergenceAnalysis analysis. For multiple blocks the lazily
evaluated inverted map VirtReg2Value was not cleared when the ValueMap map was.

[arsenm]

test/CodeGen/AMDGPU/diverge-extra-formal-args.ll

11

Move the triple to run line and delete it and the datalayout

test/CodeGen/AMDGPU/diverge-interp-mov-lower.ll

8

Ditto

[dstuttard]

Folded in triple to run line

[arsenm]

LGTM

test/CodeGen/AMDGPU/diverge-multi-func.ll

9 ↗

(On Diff #141372)

Missing check?

[dstuttard]

test/CodeGen/AMDGPU/diverge-multi-func.ll

9 ↗

(On Diff #141372)

No - this function is here purely to provoke the bug in the geometry shader, so it doesn't need a check as we don't actually care what is emitted.

[arsenm]

test/CodeGen/AMDGPU/diverge-multi-func.ll

9 ↗

(On Diff #141372)

How does this change an unrelated function?

[alex-t]

In general LGTM.
I also concerned about the magic test that has no checks and has no visible side effects on shared data but is necessary to reproduce buggy behavior.
Could you please clarify what do you need it for?

[dstuttard]

In D45372#1062947, @alex-t wrote:

In general LGTM.
I also concerned about the magic test that has no checks and has no visible side effects on shared data but is necessary to reproduce buggy behavior.
Could you please clarify what do you need it for?

Hopefully answering both Matt and Alex's concern about the magic test. The test is a bugpoint cut-down of a failing shader test (then modified by hand).
The fix that adds the VirtReg2Value.clear() is tested by this test.
This missing clear means that the lazily evaluated VirtReg2Value was being created in the first function (the untested PS shader function in this case) and then wasn't cleared for the second GS function (and hence contains the wrong mappings - for the PS function and not the GS). It doesn't really matter what the first function does, as long as it creates the VirtReg2Value mapping - the content of this function does this.
Removing the first function stops the test exposing this bug.
Hopefully this is more clear now.

I can do a couple of things if you think it worthwhile:

1. Add some more comments to the test to make it clearer why there is a "magic" untested function in the test.
2. Remove the test altogether since the VirtReg2Value.clear() is definitely required and was originally missed as an oversight - and the test might be overkill.

[alex-t]

In D45372#1068728, @dstuttard wrote:

2. Remove the test altogether since the VirtReg2Value.clear() is definitely required and was originally missed as an oversight - and the test might be overkill.

Sounds reasonable. It really was an oversight :) It's funny that the change was reviewed by lots of people for 2 months but nobody noticed not clearing state.

[dstuttard]

In D45372#1068792, @alex-t wrote:

In D45372#1068728, @dstuttard wrote:

2. Remove the test altogether since the VirtReg2Value.clear() is definitely required and was originally missed as an oversight - and the test might be overkill.

Sounds reasonable. It really was an oversight :) It's funny that the change was reviewed by lots of people for 2 months but nobody noticed not clearing state.

Ok - I agree. I'll remove that test and submit the change. Thanks.

[dstuttard]

Removed test as being overkill for the missed state clear being added (2 other
tests still remain)

lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp

 }
 
 /// clear - Clear out all the function-specific state. This returns this
 /// FunctionLoweringInfo to an empty state, ready to be used for a
 /// different function.
 void FunctionLoweringInfo::clear() {
   MBBMap.clear();
   ValueMap.clear();
+  VirtReg2Value.clear();
   StaticAllocaMap.clear();
   LiveOutRegInfo.clear();
   VisitedBBs.clear();
   ArgDbgValues.clear();
   ByValArgFrameIndexMap.clear();
   RegFixups.clear();
   RegsWithFixups.clear();
   StatepointStackSlots.clear();

lib/Target/AMDGPU/AMDGPUISelLowering.cpp

case ISD::CopyFromReg:
         const MachineRegisterInfo &MRI = MF->getRegInfo();
         const SIRegisterInfo &TRI = ST.getInstrInfo()->getRegisterInfo();
         unsigned Reg = R->getReg();
         if (TRI.isPhysicalRegister(Reg))
           return TRI.isVGPR(MRI, Reg);
 
         if (MRI.isLiveIn(Reg)) {
           // workitem.id.x workitem.id.y workitem.id.z
+          // Any VGPR formal argument is also considered divergent
           if ((MRI.getLiveInPhysReg(Reg) == AMDGPU::T0_X) ||
               (MRI.getLiveInPhysReg(Reg) == AMDGPU::T0_Y) ||
-              (MRI.getLiveInPhysReg(Reg) == AMDGPU::T0_Z)||
-              (MRI.getLiveInPhysReg(Reg) == AMDGPU::VGPR0) ||
-            (MRI.getLiveInPhysReg(Reg) == AMDGPU::VGPR1) ||
-            (MRI.getLiveInPhysReg(Reg) == AMDGPU::VGPR2))
+              (MRI.getLiveInPhysReg(Reg) == AMDGPU::T0_Z) ||
+              (TRI.isVGPR(MRI, Reg)))
               return true;
           // Formal arguments of non-entry functions
           // are conservatively considered divergent
           else if (!AMDGPU::isEntryFunctionCC(FLI->Fn->getCallingConv()))
             return true;
         }
         return !DA || DA->isDivergent(FLI->getValueFromVirtualReg(Reg));
       }
switch (N->getOpcode()) {
     {
 
     }
       return AMDGPU::isIntrinsicSourceOfDivergence(
       cast<ConstantSDNode>(N->getOperand(0))->getZExtValue());
     case ISD::INTRINSIC_W_CHAIN:
       return AMDGPU::isIntrinsicSourceOfDivergence(
       cast<ConstantSDNode>(N->getOperand(1))->getZExtValue());
+    // In some cases intrinsics that are a source of divergence have been
+    // lowered to AMDGPUISD so we also need to check those too.
+    case AMDGPUISD::INTERP_MOV:
+    case AMDGPUISD::INTERP_P1:
+    case AMDGPUISD::INTERP_P2:
+      return true;
   }
   return false;
 }
 
 //===---------------------------------------------------------------------===//
 // Target Properties
 //===---------------------------------------------------------------------===//
 

test/CodeGen/AMDGPU/diverge-extra-formal-args.ll

+; RUN: llc < %s -mtriple=amdgcn--amdpal -mcpu=tonga -verify-machineinstrs | FileCheck -check-prefixes=GCN,VI,PREGFX9 %s
+; RUN: llc < %s -mtriple=amdgcn--amdpal -mcpu=gfx810 -verify-machineinstrs | FileCheck -check-prefixes=GCN,GFX81,PREGFX9 %s
+; RUN: llc < %s -mtriple=amdgcn--amdpal -mcpu=gfx900 -verify-machineinstrs | FileCheck -check-prefixes=GCN,GFX9 %s
+
+; A test case that originally failed in divergence calculation
+; Implementation has to identify all formal args that can be a source of divergence
+
+@0 = external dso_local addrspace(4) constant [6 x <2 x float>]
+
+; GCN-LABEL: {{^}}_amdgpu_vs_main:
+; GCN-NOT: v_readfirstlane
+; PRE-GFX9: flat_load_dword
+; GFX9: global_load 
+define dllexport amdgpu_vs void @_amdgpu_vs_main(i32 inreg %arg, i32 inreg %arg1, i32 inreg %arg2, i32 inreg %arg3, i32 inreg %arg4, i32 %arg5, i32 %arg6, i32 %arg7, i32 %arg8) local_unnamed_addr #0 {
+.entry:
+  %tmp = add i32 %arg4, %arg8
+  %tmp9 = sext i32 %tmp to i64
+  %tmp10 = getelementptr [6 x <2 x float>], [6 x <2 x float>] addrspace(4)* @0, i64 0, i64 %tmp9
+  %tmp11 = load <2 x float>, <2 x float> addrspace(4)* %tmp10, align 8
+  %tmp12 = fadd nnan arcp contract <2 x float> zeroinitializer, %tmp11
+  %tmp13 = extractelement <2 x float> %tmp12, i32 1
+  call void @llvm.amdgcn.exp.f32(i32 12, i32 15, float undef, float %tmp13, float 0.000000e+00, float 1.000000e+00, i1 true, i1 false) #1
+  ret void
+}
+
+declare i64 @llvm.amdgcn.s.getpc() #0
+declare void @llvm.amdgcn.exp.f32(i32, i32, float, float, float, float, i1, i1) #1
+
+attributes #0 = { nounwind readnone speculatable }
+attributes #1 = { nounwind }

test/CodeGen/AMDGPU/diverge-interp-mov-lower.ll

+; RUN: llc < %s -mtriple=amdgcn--amdpal -mcpu=tonga -verify-machineinstrs | FileCheck -check-prefixes=GCN,VI,PREGFX9 %s
+; RUN: llc < %s -mtriple=amdgcn--amdpal -mcpu=gfx810 -verify-machineinstrs | FileCheck -check-prefixes=GCN,GFX81,PREGFX9 %s
+; RUN: llc < %s -mtriple=amdgcn--amdpal -mcpu=gfx900 -verify-machineinstrs | FileCheck -check-prefixes=GCN,GFX9 %s
+
+; Testing for failures in divergence calculations when divergent intrinsic is lowered during instruction selection
+
+@0 = external dso_local addrspace(4) constant [4 x <4 x float>]
+
+; GCN-LABEL: {{^}}_amdgpu_ps_main:
+; GCN-NOT: v_readfirstlane
+; PRE-GFX9: flat_load_dword
+; GFX9: global_load 
+define dllexport amdgpu_ps void @_amdgpu_ps_main(i32 inreg %arg) local_unnamed_addr #0 {
+.entry:
+  %tmp = call float @llvm.amdgcn.interp.mov(i32 2, i32 0, i32 0, i32 %arg) #1
+  %tmp1 = bitcast float %tmp to i32
+  %tmp2 = srem i32 %tmp1, 4
+  %tmp3 = select i1 false, i32 undef, i32 %tmp2
+  %tmp4 = sext i32 %tmp3 to i64
+  %tmp5 = getelementptr [4 x <4 x float>], [4 x <4 x float>] addrspace(4)* @0, i64 0, i64 %tmp4
+  %tmp6 = load <4 x float>, <4 x float> addrspace(4)* %tmp5, align 16
+  %tmp7 = extractelement <4 x float> %tmp6, i32 3
+  %tmp8 = call <2 x half> @llvm.amdgcn.cvt.pkrtz(float undef, float %tmp7) #1
+  call void @llvm.amdgcn.exp.compr.v2f16(i32 0, i32 15, <2 x half> undef, <2 x half> %tmp8, i1 true, i1 true) #2
+  ret void
+}
+
+declare float @llvm.amdgcn.interp.mov(i32, i32, i32, i32) #1
+declare <2 x half> @llvm.amdgcn.cvt.pkrtz(float, float) #1
+declare void @llvm.amdgcn.exp.compr.v2f16(i32, i32, <2 x half>, <2 x half>, i1, i1) #2
+
+attributes #0 = { nounwind "InitialPSInputAddr"="0" }
+attributes #1 = { nounwind readnone speculatable }
+attributes #2 = { nounwind }