AMDGPU: Fix an assertion in SIOptimizeVGPRLiveRange

[nhaehnle]

Summary

As the comment notes, the shader results in an INSERT_SUBREG with
"undef" (dead) operand in the Endif block. The register is considered
dead from a liveness analysis perspective. The correct thing to do seems
to be nothing: we keep the undef use of the register, the register
allocator should still be able to take the liveness into account
correctly.

[arsenm]

Could really use a mir test for these sorts of issues

llvm/lib/Target/AMDGPU/SIOptimizeVGPRLiveRange.cpp

535

Could this see an undef subregister def? Should probably be assert !O.readsReg() (or maybe the loop is buggy to begin with by using use_operands?)

llvm/test/CodeGen/AMDGPU/bug-deadlanes.ll

9–11

Can drop this

[ruiling]

Why I cannot reproduce the assertion? I have tried on latest version and a version several days before.

llvm/lib/Target/AMDGPU/SIOptimizeVGPRLiveRange.cpp

535

Could this see an undef subregister def?

I guess not in SSA? or could you help point out what it would look like?

I think the undef use might also appear in blocks after Endif, so the assertion on part of the cases may be not quite helpful. As we already checked the register is not live in Endif, maybe we can just replace under the check if (UseMI->isPHI() && UseBlock == Endif). But leaving some comment here still useful.

[nhaehnle]

It turns out my LLVM version was a bit old. In current main it needs -amdgpu-codegenprepare-break-large-phis=false to reproduce. And I'm adding a severely reduced MIR test.

llvm/lib/Target/AMDGPU/SIOptimizeVGPRLiveRange.cpp

535

!O.readsReg() seems to make sense, yes.

I found the loop a bit surprising as well, but I'm pretty sure it makes sense due to the way it checks for specific basic blocks (that have previously been collected taking liveness into account).

[nhaehnle]

Add MIR test, change to !O.readsReg()

[ruiling]

LGTM

llvm/lib/Target/AMDGPU/SIOptimizeVGPRLiveRange.cpp

void SIOptimizeVGPRLiveRange::optimizeLiveRange(
 
   // Replace all uses in the ELSE region or the PHIs in ENDIF block
   // Use early increment range because setReg() will update the linked list.
   for (auto &O : make_early_inc_range(MRI->use_operands(Reg))) {
     auto *UseMI = O.getParent();
     auto *UseBlock = UseMI->getParent();
     // Replace uses in Endif block
     if (UseBlock == Endif) {
-      assert(UseMI->isPHI() && "Uses should be PHI in Endif block");
+      if (UseMI->isPHI()) {
         O.setReg(NewReg);
+      } else {
+        // DetectDeadLanes may mark register uses as undef without removing
+        // them, in which case a non-phi instruction using the original register
+        // may exist in the Endif block even though the register is not live
+        // into it.
+        assert(O.isUndef());
+      }
       continue;
     }
 
     // Replace uses in Else region
     if (ElseBlocks.contains(UseBlock))
       O.setReg(NewReg);
   }
 
   // The optimized Reg is not alive through Flow blocks anymore.
   LiveVariables::VarInfo &OldVarInfo = LV->getVarInfo(Reg);

llvm/test/CodeGen/AMDGPU/bug-deadlanes.ll

+; RUN: llc -march=amdgcn -mcpu=gfx1100 -verify-machineinstrs < %s | FileCheck %s
+
+; CHECK-LABEL: {{^}}_amdgpu_ps_main:
+;
+; This test case used to hit an assertion in SIOptimizeVGPRLiveRange because of a vector with dead lanes
+; leading to an effectively dead INSERT_SUBREG.
+
+
+source_filename = "lgcPipeline"
+target datalayout = "e-p:64:64-p1:64:64-p2:32:32-p3:32:32-p4:64:64-p5:32:32-p6:32:32-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-v2048:2048-n32:64-S32-A5-G1-ni:7"
+target triple = "amdgcn--amdpal"
+
+define dllexport amdgpu_ps void @_amdgpu_ps_main(i32 %descTable2) #0 {
+.entry:
+  %i2 = zext i32 %descTable2 to i64
+  %i4 = inttoptr i64 %i2 to ptr addrspace(4)
+  %i159 = call reassoc nnan nsz arcp contract afn <4 x float> @llvm.amdgcn.image.sample.d.2d.v4f32.f32.f32(i32 15, float poison, float poison, float poison, float poison, float poison, float poison, <8 x i32> poison, <4 x i32> poison, i1 false, i32 0, i32 0)
+  %i1540 = shufflevector <4 x float> %i159, <4 x float> poison, <3 x i32> <i32 0, i32 1, i32 2>
+  %i1526 = call i32 @llvm.amdgcn.s.buffer.load.i32(<4 x i32> poison, i32 2688, i32 0)
+  %i1746 = load <4 x i32>, ptr addrspace(4) %i4, align 16
+  br label %bb1750
+
+bb1750:                                           ; preds = %bb1897, %.entry
+  %__llpc_global_proxy_r3.12.vec.extract2358295 = phi i32 [ 0, %.entry ], [ %__llpc_global_proxy_r3.12.vec.extract2358, %bb1897 ]
+  %__llpc_global_proxy_r13.20293 = phi <4 x i32> [ undef, %.entry ], [ %__llpc_global_proxy_r13.22, %bb1897 ]
+  %__llpc_global_proxy_r10.19291 = phi <4 x i32> [ poison, %.entry ], [ %i1914, %bb1897 ]
+  %i1751 = call float @llvm.amdgcn.struct.buffer.load.format.f32(<4 x i32> %i1746, i32 poison, i32 0, i32 0, i32 0)
+  %i1754 = shufflevector <4 x i32> %__llpc_global_proxy_r10.19291, <4 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 7>
+  %__llpc_global_proxy_r7.12.vec.extract1953260 = bitcast float %i1751 to i32
+  %i1760 = or i32 %__llpc_global_proxy_r7.12.vec.extract1953260, %__llpc_global_proxy_r3.12.vec.extract2358295
+  %i1786 = call i32 @llvm.amdgcn.s.buffer.load.i32(<4 x i32> poison, i32 %i1760, i32 0)
+  %__llpc_global_proxy_r11.12.vec.insert1237 = insertelement <4 x i32> %i1754, i32 %i1786, i64 3
+  %.not2783 = icmp eq i32 %i1786, 0
+  br i1 %.not2783, label %bb1789, label %bb1787
+
+bb1787:                                           ; preds = %bb1750
+  %i1788 = shufflevector <4 x i32> <i32 0, i32 0, i32 0, i32 poison>, <4 x i32> %__llpc_global_proxy_r13.20293, <4 x i32> <i32 0, i32 1, i32 2, i32 7>
+  br label %bb1897
+
+bb1789:                                           ; preds = %bb1750
+  %i1796 = call reassoc nnan nsz arcp contract afn float @llvm.amdgcn.fmed3.f32(float poison, float 0.000000e+00, float 1.000000e+00)
+  %i1797 = fmul reassoc nnan nsz arcp contract afn float %i1796, %i1796
+  %i1800 = fdiv reassoc nnan nsz arcp contract afn float %i1797, 0.000000e+00
+  %i1801 = bitcast float %i1800 to i32
+  %__llpc_global_proxy_r11.12.vec.insert1245 = insertelement <4 x i32> %__llpc_global_proxy_r11.12.vec.insert1237, i32 poison, i64 3
+  %i1818 = call reassoc nnan nsz arcp contract afn float @llvm.amdgcn.fmed3.f32(float poison, float 0.000000e+00, float 1.000000e+00)
+  %i1819 = bitcast float %i1818 to i32
+  %i1878 = shufflevector <4 x i32> %__llpc_global_proxy_r11.12.vec.insert1245, <4 x i32> poison, <3 x i32> <i32 3, i32 3, i32 3>
+  %i1879 = bitcast <3 x i32> %i1878 to <3 x float>
+  %i1881 = fmul reassoc nnan nsz arcp contract afn <3 x float> %i1540, %i1879
+  %i1882 = call <3 x i32> @llvm.amdgcn.s.buffer.load.v3i32(<4 x i32> poison, i32 poison, i32 0)
+  %i1883 = shufflevector <3 x i32> %i1882, <3 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 undef>
+  %i1884 = bitcast <4 x i32> %i1883 to <4 x float>
+  %i1885 = shufflevector <4 x float> %i1884, <4 x float> poison, <3 x i32> <i32 0, i32 1, i32 2>
+  %i1886 = insertelement <3 x i32> undef, i32 %i1819, i64 0
+  %i1887 = bitcast <3 x i32> %i1886 to <3 x float>
+  %i1888 = insertelement <3 x i32> undef, i32 %i1801, i64 0
+  %i1889 = bitcast <3 x i32> %i1888 to <3 x float>
+  %i1890 = fmul reassoc nnan nsz arcp contract afn <3 x float> %i1887, %i1889
+  %i1891 = shufflevector <3 x float> %i1890, <3 x float> poison, <3 x i32> zeroinitializer
+  %i1892 = fmul reassoc nnan nsz arcp contract afn <3 x float> %i1885, %i1891
+  %i1893 = fmul reassoc nnan nsz arcp contract afn <3 x float> %i1892, %i1881
+  %i1894 = bitcast <3 x float> %i1893 to <3 x i32>
+  %i1895 = shufflevector <3 x i32> %i1894, <3 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 undef>
+  %i1896 = insertelement <4 x i32> %i1895, i32 %i1819, i64 3
+  br label %bb1897
+
+bb1897:                                           ; preds = %bb1789, %bb1787
+  %__llpc_global_proxy_r11.19 = phi <4 x i32> [ %__llpc_global_proxy_r11.12.vec.insert1237, %bb1787 ], [ %__llpc_global_proxy_r11.12.vec.insert1245, %bb1789 ]
+  %__llpc_global_proxy_r13.22 = phi <4 x i32> [ %i1788, %bb1787 ], [ %i1896, %bb1789 ]
+  %i1898 = shufflevector <4 x i32> %__llpc_global_proxy_r11.19, <4 x i32> poison, <3 x i32> <i32 0, i32 1, i32 2>
+  %i1899 = bitcast <3 x i32> %i1898 to <3 x float>
+  %i1900 = shufflevector <4 x i32> %__llpc_global_proxy_r13.22, <4 x i32> poison, <3 x i32> <i32 0, i32 1, i32 2>
+  %i1901 = bitcast <3 x i32> %i1900 to <3 x float>
+  %i1902 = fadd reassoc nnan nsz arcp contract afn <3 x float> %i1901, %i1899
+  %i1903 = bitcast <3 x float> %i1902 to <3 x i32>
+  %i1907 = shufflevector <3 x i32> %i1903, <3 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 undef>
+  %i1908 = shufflevector <4 x i32> %i1907, <4 x i32> %__llpc_global_proxy_r11.19, <4 x i32> <i32 0, i32 1, i32 2, i32 7>
+  %i1914 = shufflevector <4 x i32> %i1908, <4 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 7>
+  %__llpc_global_proxy_r3.12.vec.extract2358 = extractelement <2 x i32> zeroinitializer, i64 1
+  %.not2780.not = icmp ult i32 %__llpc_global_proxy_r3.12.vec.extract2358, %i1526
+  br i1 %.not2780.not, label %bb1750, label %._crit_edge298
+
+._crit_edge298:                                   ; preds = %bb1897
+  ret void
+}
+
+declare <4 x float> @llvm.amdgcn.image.sample.d.2d.v4f32.f32.f32(i32 immarg, float, float, float, float, float, float, <8 x i32>, <4 x i32>, i1 immarg, i32 immarg, i32 immarg)
+declare float @llvm.amdgcn.fmed3.f32(float, float, float)
+declare float @llvm.amdgcn.struct.buffer.load.format.f32(<4 x i32>, i32, i32, i32, i32 immarg)
+declare i32 @llvm.amdgcn.s.buffer.load.i32(<4 x i32>, i32, i32 immarg)
+declare <3 x i32> @llvm.amdgcn.s.buffer.load.v3i32(<4 x i32>, i32, i32 immarg)
+
+attributes #0 = { "target-features"=",+wavefrontsize64,+cumode" }