[llvm] [RISCV][VLOPT] Move mayReadPastVL check into getMinimumVLForUser. NFC (PR #127972)

[Luke Lau via llvm-commits]

https://github.com/lukel97 created https://github.com/llvm/llvm-project/pull/127972

checkUsers currently does two things, a) work out the minimum VL read by every user and b) check that the operand info of the MI and users match.

getMinimumVLForUser handles most of a), with the exception of the check for instructions that read past VL e.g. vrgather which is still in checkUsers.

This moves it into getMinimumVLForUser to keep all that logic in one place and simplifies an upcoming patch.


>From 442071d091f0b5784a0e31363e62b0ce3d7dcc54 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Thu, 20 Feb 2025 16:52:53 +0800
Subject: [PATCH] [RISCV][VLOPT] Move mayReadPastVL check into
 getMinimumVLForUser. NFC

checkUsers currently does two things, a) work out the minimum VL read by every user and b) check that the operand info of the MI and users match.

getMinimumVLForUser handles most of a), with the exception of the check for instructions that read past VL e.g. vrgather which is still in checkUsers.

This moves it into getMinimumVLForUser to keep all that logic in one place and simplifies an upcoming patch.
---
 llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
index 8e952f5184098..b2c5253632027 100644
--- a/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
+++ b/llvm/lib/Target/RISCV/RISCVVLOptimizer.cpp
@@ -1271,6 +1271,11 @@ RISCVVLOptimizer::getMinimumVLForUser(MachineOperand &UserOp) {
     return std::nullopt;
   }
 
+  if (mayReadPastVL(UserMI)) {
+    LLVM_DEBUG(dbgs() << "    Abort because used by unsafe instruction\n");
+    return std::nullopt;
+  }
+
   unsigned VLOpNum = RISCVII::getVLOpNum(Desc);
   const MachineOperand &VLOp = UserMI.getOperand(VLOpNum);
   // Looking for an immediate or a register VL that isn't X0.
@@ -1333,11 +1338,6 @@ std::optional<MachineOperand> RISCVVLOptimizer::checkUsers(MachineInstr &MI) {
       continue;
     }
 
-    if (mayReadPastVL(UserMI)) {
-      LLVM_DEBUG(dbgs() << "    Abort because used by unsafe instruction\n");
-      return std::nullopt;
-    }
-
     auto VLOp = getMinimumVLForUser(UserOp);
     if (!VLOp)
       return std::nullopt;