[llvm] [AArch64LoadStoreOptimizer] Debug messages to track decision making. NFC (PR #77593)
Sjoerd Meijer via llvm-commits
llvm-commits at lists.llvm.org
Wed Jan 10 04:05:27 PST 2024
https://github.com/sjoerdmeijer created https://github.com/llvm/llvm-project/pull/77593
With these debug message it's possible to see why some pairs get rejected for combining.
>From 67e295cb530a5f763f900d3084e8046081c7280c Mon Sep 17 00:00:00 2001
From: Sjoerd Meijer <smeijer at nvidia.com>
Date: Tue, 9 Jan 2024 19:35:42 +0530
Subject: [PATCH] [AArch64LoadStoreOptimizer] Debug messages to track decision
making. NFC.
With these debug message it's possible to see why pairs get rejected for
combining.
---
.../AArch64/AArch64LoadStoreOptimizer.cpp | 71 ++++++++++++++-----
1 file changed, 53 insertions(+), 18 deletions(-)
diff --git a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
index b435b3ce03e7ee..dbe15fda19e41c 100644
--- a/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
+++ b/llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
@@ -1326,9 +1326,12 @@ static int alignTo(int Num, int PowOf2) {
static bool mayAlias(MachineInstr &MIa,
SmallVectorImpl<MachineInstr *> &MemInsns,
AliasAnalysis *AA) {
- for (MachineInstr *MIb : MemInsns)
- if (MIa.mayAlias(AA, *MIb, /*UseTBAA*/ false))
+ for (MachineInstr *MIb : MemInsns) {
+ if (MIa.mayAlias(AA, *MIb, /*UseTBAA*/ false)) {
+ LLVM_DEBUG(dbgs() << "Aliasing with: "; MIb->dump());
return true;
+ }
+ }
return false;
}
@@ -1757,9 +1760,11 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
// Remember any instructions that read/write memory between FirstMI and MI.
SmallVector<MachineInstr *, 4> MemInsns;
+ LLVM_DEBUG(dbgs() << "Find match for: "; FirstMI.dump());
for (unsigned Count = 0; MBBI != E && Count < Limit;
MBBI = next_nodbg(MBBI, E)) {
MachineInstr &MI = *MBBI;
+ LLVM_DEBUG(dbgs() << "Analysing 2nd insn: "; MI.dump());
UsedInBetween.accumulate(MI);
@@ -1859,6 +1864,8 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
LiveRegUnits::accumulateUsedDefed(MI, ModifiedRegUnits,
UsedRegUnits, TRI);
MemInsns.push_back(&MI);
+ LLVM_DEBUG(dbgs() << "Offset doesn't fit in immediate, "
+ << "keep looking.\n");
continue;
}
// If the alignment requirements of the paired (scaled) instruction
@@ -1868,6 +1875,9 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
LiveRegUnits::accumulateUsedDefed(MI, ModifiedRegUnits,
UsedRegUnits, TRI);
MemInsns.push_back(&MI);
+ LLVM_DEBUG(dbgs()
+ << "Offset doesn't fit due to alignment requirements, "
+ << "keep looking.\n");
continue;
}
}
@@ -1884,14 +1894,25 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
const bool SameLoadReg = MayLoad && TRI->isSuperOrSubRegisterEq(
Reg, getLdStRegOp(MI).getReg());
- // If the Rt of the second instruction was not modified or used between
- // the two instructions and none of the instructions between the second
- // and first alias with the second, we can combine the second into the
- // first.
- if (ModifiedRegUnits.available(getLdStRegOp(MI).getReg()) &&
- !(MI.mayLoad() && !SameLoadReg &&
- !UsedRegUnits.available(getLdStRegOp(MI).getReg())) &&
- !mayAlias(MI, MemInsns, AA)) {
+ // If the Rt of the second instruction (destination register of the
+ // load) was not modified or used between the two instructions and none
+ // of the instructions between the second and first alias with the
+ // second, we can combine the second into the first.
+ bool RtNotModified =
+ ModifiedRegUnits.available(getLdStRegOp(MI).getReg());
+ bool RtNotUsed = !(MI.mayLoad() && !SameLoadReg &&
+ !UsedRegUnits.available(getLdStRegOp(MI).getReg()));
+ bool DontAlias = !mayAlias(MI, MemInsns, AA);
+
+ LLVM_DEBUG(dbgs() << "Checking, can combine 2nd into 1st insn:\n"
+ << "Reg '" << getLdStRegOp(MI) << "' not modified: "
+ << (RtNotModified ? "true" : "false") << "\n"
+ << "Reg '" << getLdStRegOp(MI) << "' not used: "
+ << (RtNotUsed ? "true" : "false") << "\n"
+ << "Does not alias with other mem insns: "
+ << (DontAlias ? "true" : "false") << "\n");
+
+ if (RtNotModified && RtNotUsed && DontAlias) {
// For pairs loading into the same reg, try to find a renaming
// opportunity to allow the renaming of Reg between FirstMI and MI
// and combine MI into FirstMI; otherwise bail and keep looking.
@@ -1904,6 +1925,8 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
LiveRegUnits::accumulateUsedDefed(MI, ModifiedRegUnits,
UsedRegUnits, TRI);
MemInsns.push_back(&MI);
+ LLVM_DEBUG(dbgs() << "Can't find reg for renaming, "
+ << "keep looking.\n");
continue;
}
Flags.setRenameReg(*RenameReg);
@@ -1919,10 +1942,18 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
// between the two instructions and none of the instructions between the
// first and the second alias with the first, we can combine the first
// into the second.
- if (!(MayLoad &&
- !UsedRegUnits.available(getLdStRegOp(FirstMI).getReg())) &&
- !mayAlias(FirstMI, MemInsns, AA)) {
-
+ RtNotModified = !(
+ MayLoad && !UsedRegUnits.available(getLdStRegOp(FirstMI).getReg()));
+ DontAlias = !mayAlias(FirstMI, MemInsns, AA);
+
+ LLVM_DEBUG(dbgs() << "Checking, can combine 1st into 2nd insn:\n"
+ << "Reg '" << getLdStRegOp(FirstMI)
+ << "' not modified: "
+ << (RtNotModified ? "true" : "false") << "\n"
+ << "Does not alias with other mem insns: "
+ << (DontAlias ? "true" : "false") << "\n");
+
+ if (RtNotModified && DontAlias) {
if (ModifiedRegUnits.available(getLdStRegOp(FirstMI).getReg())) {
Flags.setMergeForward(true);
Flags.clearRenameReg();
@@ -1938,8 +1969,8 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
MBBIWithRenameReg = MBBI;
}
}
- // Unable to combine these instructions due to interference in between.
- // Keep looking.
+ LLVM_DEBUG(dbgs() << "Unable to combine these instructions due to "
+ << "interference in between, keep looking.\n");
}
}
@@ -1948,16 +1979,20 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
// If the instruction wasn't a matching load or store. Stop searching if we
// encounter a call instruction that might modify memory.
- if (MI.isCall())
+ if (MI.isCall()) {
+ LLVM_DEBUG(dbgs() << "Found a call, stop looking.\n");
return E;
+ }
// Update modified / uses register units.
LiveRegUnits::accumulateUsedDefed(MI, ModifiedRegUnits, UsedRegUnits, TRI);
// Otherwise, if the base register is modified, we have no match, so
// return early.
- if (!ModifiedRegUnits.available(BaseReg))
+ if (!ModifiedRegUnits.available(BaseReg)) {
+ LLVM_DEBUG(dbgs() << "Base reg is modified, stop looking.\n");
return E;
+ }
// Update list of instructions that read/write memory.
if (MI.mayLoadOrStore())
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