[PATCH] D125408: [riscv] Extend dataflow workaround from D119518 to fallthrough blocks

[Philip Reames via Phabricator via llvm-commits]

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We've got a lurking problem with our data flow implementation where different phases disagree, resulting in possible workarounds.  D119518 <https://reviews.llvm.org/D119518> extended a workaround, but failed to consider blocks without terminators (e.g. fallthroughs).

I have a deeper rework of the algorithm in flight over in D125232 <https://reviews.llvm.org/D125232>, but this patch is specifically a minimal fix for an active miscompile.  That change can be reworked over this once landed.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D125408

Files:
  llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
  llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.mir


Index: llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.mir
===================================================================
--- llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.mir
+++ llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.mir
@@ -674,12 +674,10 @@
   ; CHECK-NEXT:   [[PseudoVADD_VX_M1_:%[0-9]+]]:vr = PseudoVADD_VX_M1 [[PseudoVID_V_M1_]], [[PHI]], -1, 6 /* e64 */, implicit $vl, implicit $vtype
   ; CHECK-NEXT:   [[MUL:%[0-9]+]]:gpr = MUL [[PHI]], [[SRLI]]
   ; CHECK-NEXT:   [[ADD:%[0-9]+]]:gpr = ADD [[COPY]], [[MUL]]
-  ; FIXME: We insert a SEW=32,LMUL=1/2 VSETVLI here but no SEW=64,LMUL=1
-  ; VSETVLI before the VADD above. This misconfigures the VADD in the case that
-  ; the loop takes its backedge.
   ; CHECK-NEXT:   dead $x0 = PseudoVSETVLIX0 killed $x0, 87 /* e32, mf2, ta, mu */, implicit-def $vl, implicit-def $vtype, implicit $vl
   ; CHECK-NEXT:   PseudoVSE32_V_MF2 killed [[PseudoVADD_VX_M1_]], killed [[ADD]], -1, 5 /* e32 */, implicit $vl, implicit $vtype
   ; CHECK-NEXT:   [[ADDI:%[0-9]+]]:gpr = ADDI [[PHI]], 1
+  ; CHECK-NEXT:   dead $x0 = PseudoVSETVLIX0 killed $x0, 88 /* e64, m1, ta, mu */, implicit-def $vl, implicit-def $vtype, implicit $vl
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.2:
   ; CHECK-NEXT:   successors: %bb.1(0x40000000), %bb.3(0x40000000)
Index: llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
+++ llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
@@ -1176,16 +1176,19 @@
       CurInfo = VSETVLIInfo::getUnknown();
       PrevVSETVLIMI = nullptr;
     }
+  }
 
-    // If we reach the end of the block and our current info doesn't match the
-    // expected info, insert a vsetvli to correct.
-    if (!UseStrictAsserts && MI.isTerminator()) {
-      const VSETVLIInfo &ExitInfo = BlockInfo[MBB.getNumber()].Exit;
-      if (CurInfo.isValid() && ExitInfo.isValid() && !ExitInfo.isUnknown() &&
-          CurInfo != ExitInfo) {
-        insertVSETVLI(MBB, MI, ExitInfo, CurInfo);
-        CurInfo = ExitInfo;
-      }
+  // If we reach the end of the block and our current info doesn't match the
+  // expected info, insert a vsetvli to correct.
+  if (!UseStrictAsserts) {
+    const VSETVLIInfo &ExitInfo = BlockInfo[MBB.getNumber()].Exit;
+    if (CurInfo.isValid() && ExitInfo.isValid() && !ExitInfo.isUnknown() &&
+        CurInfo != ExitInfo) {
+      // Note there's an implicit assumption here that terminators never use
+      // or modify VL or VTYPE.  Also, fallthrough will return end().
+      auto InsertPt = MBB.getFirstTerminator().getInstrIterator();
+      insertVSETVLI(MBB, InsertPt, MBB.findDebugLoc(InsertPt), ExitInfo, CurInfo);
+      CurInfo = ExitInfo;
     }
   }
 


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