[llvm] [RISCV] Return nullptr for PHI defs in VSETVLIInfo::getAVLDefMI (PR #97395)

[Luke Lau via llvm-commits]

https://github.com/lukel97 created https://github.com/llvm/llvm-project/pull/97395

When checking if a VSETVLIInfo is compatible, we call hasEquallyZeroAVL when only the AVL-zeroness is demanded. This will try to lookup the defining MachineInstr (to check if it's an ADDI immediate) via getAVLDefMI, but in it we were asserting that the VSETVLIInfo's AVL wouldn't come from a phi. It turns out this can happen in normal circumstances.

This causes a crash when compiling highway, so this fixes it by relaxing the assertion.


>From 742082406b14d0850c3758aa91f04e875667424a Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Tue, 2 Jul 2024 15:47:56 +0800
Subject: [PATCH] [RISCV] Return nullptr for PHI defs in
 VSETVLIInfo::getAVLDefMI

When checking if a VSETVLIInfo is compatible, we call hasEquallyZeroAVL when only the AVL-zeroness is demanded. This will try to lookup the defining MachineInstr (to check if it's an ADDI immediate) via getAVLDefMI, but in it we were asserting that the VSETVLIInfo's AVL wouldn't come from a phi. It turns out this can happen in normal circumstances.

This causes a crash when compiling highway, so this fixes it by relaxing the assertion.
---
 llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp  |  7 ++---
 .../RISCV/rvv/vsetvli-insert-crossbb.ll       | 27 +++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp b/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
index bf693344b070a..7e6eef47c121c 100644
--- a/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp
@@ -586,13 +586,14 @@ class VSETVLIInfo {
   }
   // Most AVLIsReg infos will have a single defining MachineInstr, unless it was
   // a PHI node. In that case getAVLVNInfo()->def will point to the block
-  // boundary slot.  If LiveIntervals isn't available, then nullptr is returned.
+  // boundary slot and this will return nullptr.  If LiveIntervals isn't
+  // available, nullptr is also returned.
   const MachineInstr *getAVLDefMI(const LiveIntervals *LIS) const {
     assert(hasAVLReg());
-    if (!LIS)
+    if (!LIS || getAVLVNInfo()->isPHIDef())
       return nullptr;
     auto *MI = LIS->getInstructionFromIndex(getAVLVNInfo()->def);
-    assert(!(getAVLVNInfo()->isPHIDef() && MI));
+    assert(MI);
     return MI;
   }
 
diff --git a/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll b/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
index 68e8f5dd0a406..92c8f9611db49 100644
--- a/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/vsetvli-insert-crossbb.ll
@@ -1062,3 +1062,30 @@ exit:
   %c = call <vscale x 2 x i32> @llvm.riscv.vadd.nxv2i32(<vscale x 2 x i32> undef, <vscale x 2 x i32> %a, <vscale x 2 x i32> %d, i64 %vl)
   ret <vscale x 2 x i32> %c
 }
+
+define void @vlmax_avl_phi(i1 %cmp, ptr %p) {
+; CHECK-LABEL: vlmax_avl_phi:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    andi a0, a0, 1
+; CHECK-NEXT:    vsetvli a0, zero, e8, m1, ta, ma
+; CHECK-NEXT:    vmv.v.i v8, 0
+; CHECK-NEXT:    vsetivli zero, 1, e8, m1, ta, ma
+; CHECK-NEXT:    vse8.v v8, (a1)
+; CHECK-NEXT:    ret
+entry:
+  br i1 %cmp, label %foo, label %bar
+
+foo:
+  %vl.foo = tail call i64 @llvm.riscv.vsetvlimax.i64(i64 0, i64 0)
+  br label %exit
+
+bar:
+  %vl.bar = tail call i64 @llvm.riscv.vsetvlimax.i64(i64 0, i64 0)
+  br label %exit
+
+exit:
+  %phivl = phi i64 [ %vl.foo, %foo ], [ %vl.bar, %bar ]
+  %1 = tail call <vscale x 8 x i8> @llvm.riscv.vmv.v.x.nxv8i8.i64(<vscale x 8 x i8> poison, i8 0, i64 %phivl)
+  call void @llvm.riscv.vse.nxv8i8(<vscale x 8 x i8> %1, ptr %p, i64 1)
+  ret void
+}