[PATCH] D119973: RegAllocGreedy: Fix last chance recolor assert in impossible case

Wed Feb 16 13:26:52 PST 2022

arsenm created this revision.
arsenm added reviewers: qcolombet, MatzeB, kparzysz, foad.
Herald added subscribers: kerbowa, hiraditya, jvesely.
arsenm requested review of this revision.
Herald added a subscriber: wdng.
Herald added a project: LLVM.

This example is not compilable without handling eviction of specific
subregisters. Last chance recoloring was deciding it could try
evicting an overlapping superregister, which doesn't help make any
progress.The LiveIntervalUnion would then assert due to an overlapping
/ identical range when trying the new assignment.

      

Unfortunately this is also producing a verifier error after the
allocation fails. I've seen a number of these, and not sure if we
should just start deleting the function on error rather than trying to
figure out how to put together valid MIR.

      

I'm not super confident this is the right place to fix this. I also
have a number of failing testcases I need to fix by handling partial
evictions of superregisters.


https://reviews.llvm.org/D119973

Files:
  llvm/lib/CodeGen/RegAllocGreedy.cpp
  llvm/test/CodeGen/AMDGPU/regalloc-failure-overlapping-insert-assert.mir


Index: llvm/test/CodeGen/AMDGPU/regalloc-failure-overlapping-insert-assert.mir
===================================================================

--- /dev/null
+++ llvm/test/CodeGen/AMDGPU/regalloc-failure-overlapping-insert-assert.mir
@@ -0,0 +1,62 @@
+# RUN: not llc -march=amdgcn -mcpu=gfx908 -start-before=greedy,1 -stop-after=virtregrewriter,1 %s -o /dev/null 2>&1 | FileCheck -check-prefix=ERR %s
+# RUN: not --crash llc -march=amdgcn -mcpu=gfx908 -verify-machineinstrs -start-before=greedy,1 -stop-after=virtregrewriter,1 %s -o /dev/null 2>&1 | FileCheck -check-prefixes=ERR,VERIFIER %s
+
+# FIXME: We should not produce a verifier error after erroring
+
+# ERR: error: inline assembly requires more registers than available
+# VERIFIER: *** Bad machine code: Using an undefined physical register ***
+
+# This testcase cannot be compiled with the enforced register
+# budget. Previously, tryLastChanceRecoloring would assert here. It
+# was attempting to recolor a superregister with an overlapping
+# subregister over the same range.
+
+--- |
+  define void @foo() #0 {
+    ret void
+  }
+
+  attributes #0 = { "amdgpu-waves-per-eu"="8,8" }
+
+...
+---
+name:            foo
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: vgpr_32 }
+  - { id: 1, class: vgpr_32 }
+  - { id: 2, class: vreg_512 }
+  - { id: 3, class: vreg_256 }
+  - { id: 4, class: vreg_128 }
+  - { id: 5, class: vreg_96 }
+  - { id: 6, class: vreg_96 }
+  - { id: 7, class: vreg_512 }
+  - { id: 8, class: vreg_256 }
+  - { id: 9, class: vreg_128 }
+  - { id: 10, class: vreg_96 }
+  - { id: 11, class: vreg_96 }
+  - { id: 12, class: sreg_64 }
+  - { id: 13, class: ccr_sgpr_64 }
+  - { id: 14, class: vgpr_32 }
+machineFunctionInfo:
+  scratchRSrcReg:  '$sgpr0_sgpr1_sgpr2_sgpr3'
+  frameOffsetReg:  '$sgpr33'
+  stackPtrOffsetReg: '$sgpr32'
+body:             |
+  bb.0 (%ir-block.0):
+    liveins: $sgpr30_sgpr31
+
+    INLINEASM &"; def $0", 1 /* sideeffect attdialect */, 10 /* regdef */, implicit-def $agpr0
+    %14:vgpr_32 = COPY killed $agpr0
+    INLINEASM &"; def $0 $1 $2 $3 $4", 1 /* sideeffect attdialect */, 16842762 /* regdef:VReg_512 */, def %7, 14155786 /* regdef:VReg_256 */, def %8, 5308426 /* regdef:VReg_128 */, def %9, 3866634 /* regdef:VReg_96 */, def %10, 3866634 /* regdef:VReg_96 */, def %11
+    INLINEASM &"; clobber", 1 /* sideeffect attdialect */, 12 /* clobber */, implicit-def dead early-clobber $agpr0_agpr1_agpr2_agpr3_agpr4_agpr5_agpr6_agpr7_agpr8_agpr9_agpr10_agpr11_agpr12_agpr13_agpr14_agpr15_agpr16_agpr17_agpr18_agpr19_agpr20_agpr21_agpr22_agpr23_agpr24_agpr25_agpr26_agpr27_agpr28_agpr29_agpr30_agpr31, 12 /* clobber */, implicit-def dead early-clobber $vgpr0_vgpr1_vgpr2_vgpr3_vgpr4_vgpr5_vgpr6_vgpr7_vgpr8_vgpr9_vgpr10_vgpr11_vgpr12_vgpr13_vgpr14_vgpr15_vgpr16_vgpr17_vgpr18_vgpr19_vgpr20_vgpr21_vgpr22_vgpr23_vgpr24_vgpr25_vgpr26_vgpr27_vgpr28_vgpr29_vgpr30_vgpr31
+    INLINEASM &"; use $0", 1 /* sideeffect attdialect */, 16842761 /* reguse:VReg_512 */, %7
+    INLINEASM &"; use $0", 1 /* sideeffect attdialect */, 14155785 /* reguse:VReg_256 */, %8
+    INLINEASM &"; use $0", 1 /* sideeffect attdialect */, 5308425 /* reguse:VReg_128 */, %9
+    INLINEASM &"; use $0", 1 /* sideeffect attdialect */, 3866633 /* reguse:VReg_96 */, %10
+    INLINEASM &"; use $0", 1 /* sideeffect attdialect */, 3866633 /* reguse:VReg_96 */, %11
+    $agpr1 = COPY %14
+    INLINEASM &"; use $0", 1 /* sideeffect attdialect */, 9 /* reguse */, killed $agpr1
+    S_SETPC_B64_return killed renamable $sgpr30_sgpr31
+
+...
Index: llvm/lib/CodeGen/RegAllocGreedy.cpp
===================================================================
--- llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -1961,8 +1961,14 @@
       // it would not be recolorable as it is in the same state as VirtReg.
       // However, if VirtReg has tied defs and Intf doesn't, then
       // there is still a point in examining if it can be recolorable.
+      //
+      // Also, don't try to evict a register which is assigned to an overlapping
+      // super register.
+      //
+      // TODO: Can we evict an interfering subset of the subregisters?
       if (((ExtraInfo->getStage(*Intf) == RS_Done &&
-            MRI->getRegClass(Intf->reg()) == CurRC) &&
+            (MRI->getRegClass(Intf->reg()) == CurRC ||
+             TRI->regsOverlap(VRM->getPhys(Intf->reg()), PhysReg))) &&
            !(hasTiedDef(MRI, VirtReg.reg()) &&
              !hasTiedDef(MRI, Intf->reg()))) ||
           FixedRegisters.count(Intf->reg())) {


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