[llvm] [MachineCombiner] Don't ignore PHI depths (PR #82025)

[Jonas Paulsson via llvm-commits]

https://github.com/JonPsson1 created https://github.com/llvm/llvm-project/pull/82025

The depths of the Root and the NewRoot are to be compared in MachineCombiner::improvesCriticalPathLen(), and while the call to BlockTrace.getInstrCycles(*Root) includes the Depth of a PHI, for some reason PHI nodes have been ignored in getOperandDef(). This seems wrong - according to the comment there there should be no depth info for PHIs, but the trace debug shows:

```
Depths for %bb.2:
     48 Instructions
15      13      %52:gpr = PHI %30:gpr, %bb.0, %51:gpr, %bb.1
15      7       %53:gpr = SLTU %40:gpr, %39:gpr
...
```
This means that Root currenty has that depth included in it, but NewRoot ignores it. My guess is that MachineTraceMetrics has been improved and this was missed in MachineCombiner. This patch simply removes that check for PHIs so that they are also counted.

@LuoYuanke @asi-sc @preames @uweigand 

>From 9af24a85014d87d8648674beeef357d57ab075a2 Mon Sep 17 00:00:00 2001
From: Jonas Paulsson <paulson1 at linux.ibm.com>
Date: Fri, 16 Feb 2024 19:36:44 +0100
Subject: [PATCH] Don't ignore PHI depths

---
 llvm/lib/CodeGen/MachineCombiner.cpp                      | 3 ---
 .../test/CodeGen/RISCV/umulo-128-legalisation-lowering.ll | 4 ++--
 llvm/test/CodeGen/X86/bitcast-and-setcc-256.ll            | 8 ++++----
 3 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/CodeGen/MachineCombiner.cpp b/llvm/lib/CodeGen/MachineCombiner.cpp
index c65937935ed820..a4c87a7678bd8d 100644
--- a/llvm/lib/CodeGen/MachineCombiner.cpp
+++ b/llvm/lib/CodeGen/MachineCombiner.cpp
@@ -155,9 +155,6 @@ MachineCombiner::getOperandDef(const MachineOperand &MO) {
   // We need a virtual register definition.
   if (MO.isReg() && MO.getReg().isVirtual())
     DefInstr = MRI->getUniqueVRegDef(MO.getReg());
-  // PHI's have no depth etc.
-  if (DefInstr && DefInstr->isPHI())
-    DefInstr = nullptr;
   return DefInstr;
 }
 
diff --git a/llvm/test/CodeGen/RISCV/umulo-128-legalisation-lowering.ll b/llvm/test/CodeGen/RISCV/umulo-128-legalisation-lowering.ll
index 8c0d97afe6c21d..f1ae3200175636 100644
--- a/llvm/test/CodeGen/RISCV/umulo-128-legalisation-lowering.ll
+++ b/llvm/test/CodeGen/RISCV/umulo-128-legalisation-lowering.ll
@@ -89,8 +89,8 @@ define { i128, i8 } @muloti_test(i128 %l, i128 %r) #0 {
 ; RISCV32-NEXT:    snez a3, a3
 ; RISCV32-NEXT:    and a3, a3, a7
 ; RISCV32-NEXT:    or a2, a3, a2
-; RISCV32-NEXT:    or a3, t2, t3
-; RISCV32-NEXT:    or a2, a2, a3
+; RISCV32-NEXT:    or a2, a2, t2
+; RISCV32-NEXT:    or a2, a2, t3
 ; RISCV32-NEXT:    mul a3, a5, a4
 ; RISCV32-NEXT:    andi a2, a2, 1
 ; RISCV32-NEXT:    sw a3, 0(a0)
diff --git a/llvm/test/CodeGen/X86/bitcast-and-setcc-256.ll b/llvm/test/CodeGen/X86/bitcast-and-setcc-256.ll
index 34ef23db345755..234c7a0a500d30 100644
--- a/llvm/test/CodeGen/X86/bitcast-and-setcc-256.ll
+++ b/llvm/test/CodeGen/X86/bitcast-and-setcc-256.ll
@@ -553,8 +553,8 @@ define i8 @v8i32_or_select(<8 x i32> %a0, <8 x i32> %a1, <8 x i32> %a2, <8 x i32
 ; AVX1-NEXT:    vpcmpeqd %xmm5, %xmm4, %xmm4
 ; AVX1-NEXT:    vpcmpeqd %xmm1, %xmm0, %xmm0
 ; AVX1-NEXT:    vinsertf128 $1, %xmm4, %ymm0, %ymm0
-; AVX1-NEXT:    vorps %ymm2, %ymm3, %ymm1
-; AVX1-NEXT:    vorps %ymm0, %ymm1, %ymm0
+; AVX1-NEXT:    vorps %ymm0, %ymm3, %ymm0
+; AVX1-NEXT:    vorps %ymm2, %ymm0, %ymm0
 ; AVX1-NEXT:    vmovmskps %ymm0, %eax
 ; AVX1-NEXT:    # kill: def $al killed $al killed $eax
 ; AVX1-NEXT:    vzeroupper
@@ -571,8 +571,8 @@ define i8 @v8i32_or_select(<8 x i32> %a0, <8 x i32> %a1, <8 x i32> %a2, <8 x i32
 ; AVX2-NEXT:    vpcmpeqd %ymm2, %ymm0, %ymm2
 ; AVX2-NEXT:  .LBB7_3:
 ; AVX2-NEXT:    vpcmpeqd %ymm1, %ymm0, %ymm0
-; AVX2-NEXT:    vpor %ymm2, %ymm3, %ymm1
-; AVX2-NEXT:    vpor %ymm0, %ymm1, %ymm0
+; AVX2-NEXT:    vpor %ymm0, %ymm3, %ymm0
+; AVX2-NEXT:    vpor %ymm2, %ymm0, %ymm0
 ; AVX2-NEXT:    vmovmskps %ymm0, %eax
 ; AVX2-NEXT:    # kill: def $al killed $al killed $eax
 ; AVX2-NEXT:    vzeroupper