[llvm] b832c49 - [RISCV] Fix VLOptimizer assert, relax ElementsDependOn on viota/vms{b,i,o}f.m (#149698)

[via llvm-commits]

Author: Luke Lau
Date: 2025-07-21T14:51:41+08:00
New Revision: b832c49cb4d7668e6ab49c984ba3f4c56356f023

URL: https://github.com/llvm/llvm-project/commit/b832c49cb4d7668e6ab49c984ba3f4c56356f023
DIFF: https://github.com/llvm/llvm-project/commit/b832c49cb4d7668e6ab49c984ba3f4c56356f023.diff

LOG: [RISCV] Fix VLOptimizer assert, relax ElementsDependOn on viota/vms{b,i,o}f.m (#149698)

The previous assert wasn't passing the TSFlags but the opcode, so wasn't
working.

Fixing it reveals that it was actually triggering, because we're too
strict with viota and vmsxf.m We already reduce the VL on these
instructions because the result in each element doesn't depend on VL.
However, it does change if masked, so account for that.

Added: 
    

Modified: 
    llvm/lib/Target/RISCV/RISCVInstrFormats.td
    llvm/lib/Target/RISCV/RISCVInstrInfoV.td
    llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/RISCV/RISCVInstrFormats.td b/llvm/lib/Target/RISCV/RISCVInstrFormats.td
index e23001a3a0bff..d9c6101478064 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrFormats.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrFormats.td
@@ -174,6 +174,7 @@ class EltDeps<bit vl, bit mask> {
 
 def EltDepsNone      : EltDeps<vl=0, mask=0>;
 def EltDepsVL        : EltDeps<vl=1, mask=0>;
+def EltDepsMask      : EltDeps<vl=0, mask=1>;
 def EltDepsVLMask    : EltDeps<vl=1, mask=1>;
 
 class EEW <bits<2> val> {

diff  --git a/llvm/lib/Target/RISCV/RISCVInstrInfoV.td b/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
index 5d13a877c90a2..33c713833d8b9 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfoV.td
@@ -1642,7 +1642,7 @@ def VFIRST_M : RVInstV<0b010000, 0b10001, OPMVV, (outs GPR:$vd),
 
 def : MnemonicAlias<"vpopc.m", "vcpop.m">;
 
-let Constraints = "@earlyclobber $vd", RVVConstraint = Iota, ElementsDependOn = EltDepsVLMask in {
+let Constraints = "@earlyclobber $vd", RVVConstraint = Iota, ElementsDependOn = EltDepsMask in {
 
 let DestEEW = EEW1 in {
 // vmsbf.m set-before-first mask bit
@@ -1655,7 +1655,7 @@ defm VMSOF_M : VMSFS_MV_V<"vmsof.m", 0b010100, 0b00010>;
 // Vector Iota Instruction
 defm VIOTA_M : VIOTA_MV_V<"viota.m", 0b010100, 0b10000>;
 
-} // Constraints = "@earlyclobber $vd", RVVConstraint = Iota, ElementsDependOn = EltDepsVLMask
+} // Constraints = "@earlyclobber $vd", RVVConstraint = Iota, ElementsDependOn = EltDepsMask
 
 // Vector Element Index Instruction
 let hasSideEffects = 0, mayLoad = 0, mayStore = 0 in {

diff  --git a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
index e656e8bb99d86..15bd3466373a7 100644
--- a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
+++ b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
@@ -33,6 +33,7 @@ namespace {
 class RISCVVLOptimizer : public MachineFunctionPass {
   const MachineRegisterInfo *MRI;
   const MachineDominatorTree *MDT;
+  const TargetInstrInfo *TII;
 
 public:
   static char ID;
@@ -1291,7 +1292,8 @@ bool RISCVVLOptimizer::isCandidate(const MachineInstr &MI) const {
     return false;
   }
 
-  assert(!RISCVII::elementsDependOnVL(RISCV::getRVVMCOpcode(MI.getOpcode())) &&
+  assert(!RISCVII::elementsDependOnVL(
+             TII->get(RISCV::getRVVMCOpcode(MI.getOpcode())).TSFlags) &&
          "Instruction shouldn't be supported if elements depend on VL");
 
   assert(MI.getOperand(0).isReg() &&
@@ -1495,6 +1497,8 @@ bool RISCVVLOptimizer::runOnMachineFunction(MachineFunction &MF) {
   if (!ST.hasVInstructions())
     return false;
 
+  TII = ST.getInstrInfo();
+
   // For each instruction that defines a vector, compute what VL its
   // downstream users demand.
   for (MachineBasicBlock *MBB : post_order(&MF)) {