[llvm] a95edec - [RISCV] Use vleff's AVL when output VL doesn't dominate in RISCVVLOptimizer (#156618)

[via llvm-commits]

Author: Luke Lau
Date: 2025-09-04T11:56:29+08:00
New Revision: a95edec28a4900d15f6fce17561964071600609a

URL: https://github.com/llvm/llvm-project/commit/a95edec28a4900d15f6fce17561964071600609a
DIFF: https://github.com/llvm/llvm-project/commit/a95edec28a4900d15f6fce17561964071600609a.diff

LOG: [RISCV] Use vleff's AVL when output VL doesn't dominate in RISCVVLOptimizer (#156618)

If an instruction's demanded VL is a virtual register defined by a vleff
instruction, it might not dominate and fail to have its VL reduced.

In leiu of the output VL, we can try and use the AVL passed to the vleff
itself since it will be at least greater than or equal the original VL.

I tried to create an LLVM IR test for this in but didn't have any luck
because the scheduler kept on moving the instruction past the vleff, so
it always dominated. So I've just included some mir tests instead.

Added: 
    

Modified: 
    llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
    llvm/test/CodeGen/RISCV/rvv/vl-opt.mir

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
index 9b70eb6c25b12..baf6aaac62779 100644
--- a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
+++ b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
@@ -1464,6 +1464,15 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &MI) const {
   assert((CommonVL->isImm() || CommonVL->getReg().isVirtual()) &&
          "Expected VL to be an Imm or virtual Reg");
 
+  // If the VL is defined by a vleff that doesn't dominate MI, try using the
+  // vleff's AVL. It will be greater than or equal to the output VL.
+  if (CommonVL->isReg()) {
+    const MachineInstr *VLMI = MRI->getVRegDef(CommonVL->getReg());
+    if (RISCVInstrInfo::isFaultOnlyFirstLoad(*VLMI) &&
+        !MDT->dominates(VLMI, &MI))
+      CommonVL = VLMI->getOperand(RISCVII::getVLOpNum(VLMI->getDesc()));
+  }
+
   if (!RISCV::isVLKnownLE(*CommonVL, VLOp)) {
     LLVM_DEBUG(dbgs() << "  Abort due to CommonVL not <= VLOp.\n");
     return false;

diff  --git a/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir b/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
index 60398cdf1db66..0acdca91ee84c 100644
--- a/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
+++ b/llvm/test/CodeGen/RISCV/rvv/vl-opt.mir
@@ -603,4 +603,61 @@ body: |
     $x10 = COPY %9
     PseudoRET implicit $x10
 ...
-
+---
+name: vleff_imm
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: vleff_imm
+    ; CHECK: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
+    %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
+    PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+...
+---
+name: vleff_reg_dominates
+body: |
+  bb.0:
+    liveins: $x8
+    ; CHECK-LABEL: name: vleff_reg_dominates
+    ; CHECK: liveins: $x8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %avl:gprnox0 = COPY $x8
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, %avl, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, %avl, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+    %avl:gprnox0 = COPY $x8
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
+    %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, %avl, 3 /* e8 */, 3 /* ta, ma */
+    PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+...
+---
+name: vleff_reg_doesnt_dominate
+body: |
+  bb.0:
+    liveins: $x8
+    ; CHECK-LABEL: name: vleff_reg_doesnt_dominate
+    ; CHECK: liveins: $x8
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: %avl:gprnox0 = COPY $x8
+    ; CHECK-NEXT: %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, %avl, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
+    %avl:gprnox0 = COPY $x8
+    %y:vr, %vl:gprnox0 = PseudoVLE8FF_V_M1 $noreg, $noreg, %avl, 3 /* e8 */, 3 /* ta, ma */
+    PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+...
+---
+name: vleff_mask_imm
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: vleff_mask_imm
+    ; CHECK: %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: %y:vrnov0, %vl:gprnox0 = PseudoVLE8FF_V_M1_MASK $noreg, $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
+    ; CHECK-NEXT: PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+    %x:vr = PseudoVADD_VV_M1 $noreg, $noreg, $noreg, -1, 3 /* e8 */, 3 /* ta, ma */
+    %y:vrnov0, %vl:gprnox0 = PseudoVLE8FF_V_M1_MASK $noreg, $noreg, $noreg, 1, 3 /* e8 */, 3 /* ta, ma */
+    PseudoVSE8_V_M1 %x, $noreg, %vl, 3 /* e8 */
+...