[llvm] [RISCV][VLOPT] Look through PHI instructions (PR #132236)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland created https://github.com/llvm/llvm-project/pull/132236

Similar to what we do for copies. We may reduce one of the PHI operands and not the other, and thats perfectly okay.

>From 2513c597fe77945b6c4bbe6c0e570c8ca4458fa5 Mon Sep 17 00:00:00 2001
From: Michael Maitland <michaeltmaitland at gmail.com>
Date: Thu, 20 Mar 2025 08:15:19 -0700
Subject: [PATCH] [RISCV][VLOPT] Look through PHI instructions

---
 llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp |  7 ++
 llvm/test/CodeGen/RISCV/rvv/vl-opt.mir     | 84 ++++++++++++++++++++++
 2 files changed, 91 insertions(+)

diff --git a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
index c36a1e9adccb0..61f43e2e72b13 100644
--- a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
+++ b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
@@ -1340,6 +1340,13 @@ RISCVVLOptimizer::checkUsers(const MachineInstr &MI) const {
       continue;
     }
 
+    if (UserMI.isPHI()) {
+      LLVM_DEBUG(dbgs() << "    Peeking through uses of PHI\n");
+        for (auto &PhiUse : MRI->use_operands(UserMI.getOperand(0).getReg()))
+          Worklist.insert(&PhiUse);
+      continue;
+    }
+
     auto VLOp = getMinimumVLForUser(UserOp);
     if (!VLOp)
       return std::nullopt;
diff --git a/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir b/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
index d42feeca9dbcc..dfc4dd84c72b2 100644
--- a/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
+++ b/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
@@ -368,3 +368,87 @@ body: |
     %y:vr = COPY %x
     %z:vr = PseudoVADD_VV_M1 $noreg, %y, $noreg, 1, 4 /* e16 */, 0 /* tu, mu */
 ...
+---
+name: phi
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: phi
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x40000000), %bb.1(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %w:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+  ; CHECK-NEXT:   BNE $noreg, $noreg, %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   %y:vr = PHI %w, %bb.0, %x, %bb.1
+  ; CHECK-NEXT:   %z:vr = PseudoVADD_VV_M1 $noreg, %y, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+  bb.0:
+    %w:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+    BNE $noreg, $noreg, %bb.2
+  bb.1:
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+  bb.2:
+    %y:vr = PHI %w, %bb.0, %x, %bb.1
+    %z:vr = PseudoVADD_VV_M1 $noreg, %y, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+...
+---
+name: phi_user_invalid_sew
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: phi_user_invalid_sew
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x40000000), %bb.1(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %w:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+  ; CHECK-NEXT:   BNE $noreg, $noreg, %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   %y:vr = PHI %w, %bb.0, %x, %bb.1
+  ; CHECK-NEXT:   %z:vr = PseudoVADD_VV_M1 $noreg, %y, $noreg, 1, 4 /* e16 */, 0 /* tu, mu */
+  bb.0:
+    %w:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+    BNE $noreg, $noreg, %bb.2
+  bb.1:
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+  bb.2:
+    %y:vr = PHI %w, %bb.0, %x, %bb.1
+    %z:vr = PseudoVADD_VV_M1 $noreg, %y, $noreg, 1, 4 /* e16 */, 0 /* tu, mu */
+...
+---
+name: phi_different_incoming_sew
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: phi_different_incoming_sew
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.2(0x40000000), %bb.1(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %w:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+  ; CHECK-NEXT:   BNE $noreg, $noreg, %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.2(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 4 /* e16 */, 0 /* tu, mu */
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   %y:vr = PHI %w, %bb.0, %x, %bb.1
+  ; CHECK-NEXT:   %z:vr = PseudoVADD_VV_M1 $noreg, %y, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+  bb.0:
+    %w:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 0 /* tu, mu */
+    BNE $noreg, $noreg, %bb.2
+  bb.1:
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 4 /* e16 */, 0 /* tu, mu */
+  bb.2:
+    %y:vr = PHI %w, %bb.0, %x, %bb.1
+    %z:vr = PseudoVADD_VV_M1 $noreg, %y, $noreg, 1, 3 /* e8 */, 0 /* tu, mu */
+...