[llvm] [MachinePipeliner] Skip reserved registers when computing register pressure (PR #120694)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp created https://github.com/llvm/llvm-project/pull/120694

We used to skip fixed registers, but fixed registers are not enough
because there are some runtime unusable registers like registers
reserved by `-ffixed-xxx` options.

Here we change to use reserved registers so that the estimated
pressure is more accurate.


>From c9c9ff0b2fc84d47459a5e69e5f89683dc596c49 Mon Sep 17 00:00:00 2001
From: Wang Pengcheng <wangpengcheng.pp at bytedance.com>
Date: Fri, 20 Dec 2024 16:09:31 +0800
Subject: [PATCH] [MachinePipeliner] Skip reserved registers when computing
 register pressure

We used to skip fixed registers, but fixed registers are not enough
because there are some runtime unusable registers like registers
reserved by `-ffixed-xxx` options.

Here we change to use reserved registers so that the estimated
pressure is more accurate.
---
 llvm/lib/CodeGen/MachinePipeliner.cpp | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/CodeGen/MachinePipeliner.cpp b/llvm/lib/CodeGen/MachinePipeliner.cpp
index d2eb7e72ac6b2a..db12b9c343b6bb 100644
--- a/llvm/lib/CodeGen/MachinePipeliner.cpp
+++ b/llvm/lib/CodeGen/MachinePipeliner.cpp
@@ -1283,9 +1283,9 @@ class HighRegisterPressureDetector {
     }
   }
 
-  // Return true if Reg is fixed one, for example, stack pointer
-  bool isFixedRegister(Register Reg) const {
-    return Reg.isPhysical() && TRI->isFixedRegister(MF, Reg.asMCReg());
+  // Return true if Reg is reserved one, for example, stack pointer
+  bool isReservedRegister(Register Reg) const {
+    return Reg.isPhysical() && MRI.isReserved(Reg.asMCReg());
   }
 
   bool isDefinedInThisLoop(Register Reg) const {
@@ -1311,7 +1311,7 @@ class HighRegisterPressureDetector {
         // because it's used only at the first iteration.
         if (MI.isPHI() && Reg != getLoopPhiReg(MI, OrigMBB))
           continue;
-        if (isFixedRegister(Reg))
+        if (isReservedRegister(Reg))
           continue;
         if (isDefinedInThisLoop(Reg))
           continue;
@@ -1423,7 +1423,7 @@ class HighRegisterPressureDetector {
 
     const auto InsertReg = [this, &CurSetPressure](RegSetTy &RegSet,
                                                    Register Reg) {
-      if (!Reg.isValid() || isFixedRegister(Reg))
+      if (!Reg.isValid() || isReservedRegister(Reg))
         return;
 
       bool Inserted = RegSet.insert(Reg).second;
@@ -1437,7 +1437,7 @@ class HighRegisterPressureDetector {
 
     const auto EraseReg = [this, &CurSetPressure](RegSetTy &RegSet,
                                                   Register Reg) {
-      if (!Reg.isValid() || isFixedRegister(Reg))
+      if (!Reg.isValid() || isReservedRegister(Reg))
         return;
 
       // live-in register