[llvm] [MacroFusion][RISCV] Allocate same register for second instruction of fusible pair (PR #77461)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Wang Pengcheng (wangpc-pp)

<details>
<summary>Changes</summary>

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
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)

``````````

</details>


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