[MacroFusion][RISCV] Allocate same register for second instruction of fusible pair

[wangpc-pp]

We add a MI flag to indicate the constraint and set this flag to
true for the second instruction of fusible pairs in pre-regalloc
macrofusion.

Then, we add register allocation hints for it.

During regalloc, the allocator will choose the same register
according to the hint.

This is a PoC currently.

[mgudim]

don't we need to check that the "first" instuction is part of a fusible pair?

[wangpc-pp]

I think we don't need to, because this has been done in MacroFusion mutation?

[mgudim]

Is the point to add a flag to simplify the code which detects fusible patterns? I realise that some processors may have a lot of fusions, but is adding a new flag really the right solution?

[wangpc-pp]

Is the point to add a flag to simplify the code which detects fusible patterns?

Yes!

I realise that some processors may have a lot of fusions, but is adding a new flag really the right solution?

I think this can be feasible, there are already some values in MIFlag that are not so common, like NoConvergent (SIMT only I think), Unpredictable (X86 only), etc.

[topperc]

What if the second fusion instruction is an instruction with 2 register sources instead of 1 register and an immediate. We'll need to know which operand to constrain.

[wangpc-pp]

What if the second fusion instruction is an instruction with 2 register sources

Currently, I think this is a uncommon case and I don't know if there are such fusions. Do you have any example? Maybe integer multiply-add fusion?
Even if so, I think we can know the constrainted operands via its opcode and constrain them in the getRegAllocationHints implementation. We don't think we need to record this info in target-independent part.

[topperc]

From the RISC-V section of this page https://en.wikichip.org/wiki/macro-operation_fusion which are also listed here https://xiangshan-doc.readthedocs.io/zh-cn/latest/frontend/decode/

slli rd, rs1, {1,2,3}
add rd, rd, rs2

There are a few others on https://xiangshan-doc.readthedocs.io/zh-cn/latest/frontend/decode/

[wangpc-pp]

Thanks, I didn't take a deep look into XiangShan's fusions (facepalming).

[llvmbot]

@llvm/pr-subscribers-backend-risc-v

Author: Wang Pengcheng (wangpc-pp)

Changes

We add a MI flag to indicate the constraint and set this flag to
true for the second instruction of fusible pairs in pre-regalloc
macrofusion.

Then, we add register allocation hints for it.

During regalloc, the allocator will choose the same register
according to the hint.

This is a PoC currently.

---

Full diff: https://github.com/llvm/llvm-project/pull/77461.diff

4 Files Affected:

• (modified) llvm/include/llvm/CodeGen/MachineInstr.h (+4)
• (modified) llvm/lib/CodeGen/MacroFusion.cpp (+7)
• (modified) llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp (+2)
• (added) llvm/test/CodeGen/RISCV/pr76779.ll (+37)

diff --git a/llvm/include/llvm/CodeGen/MachineInstr.h b/llvm/include/llvm/CodeGen/MachineInstr.h
index bd72ac23fc9c08..f694c27a3f7f7f 100644
--- a/llvm/include/llvm/CodeGen/MachineInstr.h
+++ b/llvm/include/llvm/CodeGen/MachineInstr.h
@@ -114,6 +114,7 @@ class MachineInstr
                              // this instruction.
     Unpredictable = 1 << 16, // Instruction with unpredictable condition.
     NoConvergent = 1 << 17,  // Call does not require convergence guarantees.
+    Fusible = 1 << 18,       // Instruction is the second of a fusible pair.
   };
 
 private:
@@ -1030,6 +1031,9 @@ class MachineInstr
     return hasProperty(MCID::Convergent, Type);
   }
 
+  /// Return true if this instruction is fusible.
+  bool isFusible() const { return getFlag(Fusible); }
+
   /// Returns true if the specified instruction has a delay slot
   /// which must be filled by the code generator.
   bool hasDelaySlot(QueryType Type = AnyInBundle) const {
diff --git a/llvm/lib/CodeGen/MacroFusion.cpp b/llvm/lib/CodeGen/MacroFusion.cpp
index 5bd6ca0978a4b1..e4aa4636442db2 100644
--- a/llvm/lib/CodeGen/MacroFusion.cpp
+++ b/llvm/lib/CodeGen/MacroFusion.cpp
@@ -13,6 +13,7 @@
 
 #include "llvm/CodeGen/MacroFusion.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/ScheduleDAG.h"
 #include "llvm/CodeGen/ScheduleDAGInstrs.h"
@@ -128,6 +129,12 @@ bool llvm::fuseInstructionPair(ScheduleDAGInstrs &DAG, SUnit &FirstSU,
     }
   }
 
+  // Mark the second instruction of fusible pair as MachineInstr::Fusible if
+  // this mutation is running in pre-ra scheduler.
+  if (!DAG.MF.getProperties().hasProperty(
+          MachineFunctionProperties::Property::NoVRegs))
+    SecondSU.getInstr()->setFlag(MachineInstr::Fusible);
+
   ++NumFused;
   return true;
 }
diff --git a/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp b/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
index 24f8d600f1eafc..741bb51d069437 100644
--- a/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp
@@ -823,6 +823,8 @@ bool RISCVRegisterInfo::getRegAllocationHints(
         tryAddHint(MO, MI.getOperand(0), NeedGPRC);
       }
     }
+    if (MI.isFusible() && OpIdx == 1)
+      tryAddHint(MO, MI.getOperand(0), false);
   }
 
   for (MCPhysReg OrderReg : Order)
diff --git a/llvm/test/CodeGen/RISCV/pr76779.ll b/llvm/test/CodeGen/RISCV/pr76779.ll
new file mode 100644
index 00000000000000..cec132251e3c4c
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/pr76779.ll
@@ -0,0 +1,37 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+;RUN: llc < %s -mtriple=riscv64 -mattr=+f -target-abi=lp64f \
+;RUN:   | FileCheck %s --check-prefix=NOFUSION
+;RUN: llc < %s -mtriple=riscv64 -mattr=+f,+lui-addi-fusion \
+;RUN:   -target-abi=lp64f | FileCheck %s --check-prefix=FUSION
+;RUN: llc < %s -mtriple=riscv64 -mattr=+f,+lui-addi-fusion,+use-postra-scheduler \
+;RUN:   -target-abi=lp64f | FileCheck %s --check-prefixes=FUSION-POSTRA
+
+define void @foo(i32 noundef signext %0, i32 noundef signext %1) {
+; NOFUSION-LABEL: foo:
+; NOFUSION:       # %bb.0:
+; NOFUSION-NEXT:    lui a0, 3014
+; NOFUSION-NEXT:    addiw a2, a0, 334
+; NOFUSION-NEXT:    mv a0, a1
+; NOFUSION-NEXT:    mv a1, a2
+; NOFUSION-NEXT:    tail bar
+;
+; FUSION-LABEL: foo:
+; FUSION:       # %bb.0:
+; FUSION-NEXT:    lui a2, 3014
+; FUSION-NEXT:    addiw a2, a2, 334
+; FUSION-NEXT:    mv a0, a1
+; FUSION-NEXT:    mv a1, a2
+; FUSION-NEXT:    tail bar
+;
+; FUSION-POSTRA-LABEL: foo:
+; FUSION-POSTRA:       # %bb.0:
+; FUSION-POSTRA-NEXT:    lui a2, 3014
+; FUSION-POSTRA-NEXT:    addiw a2, a2, 334
+; FUSION-POSTRA-NEXT:    mv a0, a1
+; FUSION-POSTRA-NEXT:    mv a1, a2
+; FUSION-POSTRA-NEXT:    tail bar
+  tail call void @bar(i32 noundef signext %1, i32 noundef signext 12345678)
+  ret void
+}
+
+declare void @bar(i32 noundef signext, i32 noundef signext)