AMDGPU: Fix crash when folding immediates into multiple uses

[nhaehnle]

Summary

When an immediate is folded by constant folding, we re-scan the entire
use list for two reasons:

1. The constant folding may have created a new use of the same reg.
2. The constant folding may have removed an additional use in the list we're currently traversing (e.g., constant folding an S_ADD_I32 c, c).

However, this could previously lead to a crash when an unrelated use was
added twice into the FoldList. Since we re-scan the whole list anyway, we
might as well just clear the FoldList again before we do so.

Using a MIR test to show this because real code seems to trigger the issue
only in connection with some really subtle control flow structures.

Fixes GL45-CTS.shading_language_420pack.binding_images on gfx9.

[arsenm]

LGTM but test can be reduced a little bit

test/CodeGen/MIR/AMDGPU/fold-multiple.mir

34–47

You can delete this

56

You can remove the MMO, the name of the block and then you can remove the IR section

lib/Target/AMDGPU/SIFoldOperands.cpp

for (MachineRegisterInfo::use_iterator
       if (OpToFold.isImm() && tryConstantFoldOp(*MRI, TII, UseMI, &OpToFold)) {
         DEBUG(dbgs() << "Constant folded " << *UseMI <<'\n');
 
         // Some constant folding cases change the same immediate's use to a new
         // instruction, e.g. and x, 0 -> 0. Make sure we re-visit the user
         // again. The same constant folded instruction could also have a second
         // use operand.
         NextUse = MRI->use_begin(Dst.getReg());
+        FoldList.clear();
         continue;
       }
 
       // Try to fold any inline immediate uses, and then only fold other
       // constants if they have one use.
       //
       // The legality of the inline immediate must be checked based on the use
       // operand, not the defining instruction, because 32-bit instructions

test/CodeGen/MIR/AMDGPU/fold-multiple.mir

+# RUN: llc --mtriple=amdgcn--amdhsa -mcpu=fiji -verify-machineinstrs -run-pass si-fold-operands,si-shrink-instructions %s -o - | FileCheck %s
+--- |
+  define amdgpu_kernel void @test() #0 {
+    ret void
+  }
+
+  attributes #0 = { nounwind }
+
+...
+---
+
+# This used to crash / trigger an assertion, because re-scanning the use list
+# after constant-folding the definition of %3 lead to the definition of %2
+# being processed twice.
+
+# CHECK-LABEL: name: test
+# CHECK: %2 = V_LSHLREV_B32_e32 2, killed %0, implicit %exec
+# CHECK: %4 = V_AND_B32_e32 8, killed %2, implicit %exec
+
+name:            test
+alignment:       0
+exposesReturnsTwice: false
+legalized:       false
+regBankSelected: false
+selected:        false
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: vgpr_32 }
+  - { id: 1, class: sreg_32 }
+  - { id: 2, class: vgpr_32 }
+  - { id: 3, class: sreg_32 }
+  - { id: 4, class: vgpr_32 }
+  - { id: 5, class: sreg_128 }
+frameInfo:
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        false
+  maxCallFrameSize: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+body:             |
+  bb.0 (%ir-block.0):
+    %0 = IMPLICIT_DEF
+    %1 = S_MOV_B32 2
+    %2 = V_LSHLREV_B32_e64 %1, killed %0, implicit %exec
+    %3 = S_LSHL_B32 %1, killed %1, implicit-def dead %scc
+    %4 = V_AND_B32_e64 killed %2, killed %3, implicit %exec
+    %5 = IMPLICIT_DEF
+    BUFFER_STORE_DWORD_OFFSET killed %4, killed %5, 0, 0, 0, 0, 0, implicit %exec :: (volatile store 2 into `half addrspace(1)* undef`)
+    S_ENDPGM
+
+...