[llvm][CodeGen] Fixed a bug in stall cycle calculation for window scheduler

[kaiyan96]

Fixed a bug in stall cycle calculation.
When a register defined by an instruction in the current iteration is used by an instruction in the next iteration, we have modified the number of stall cycle that need to be inserted.

[llvmbot]

@llvm/pr-subscribers-backend-hexagon

Author: Kai Yan (kaiyan96)

Changes

Fixed a bug in stall cycle calculation.
When a register defined by an instruction in the current iteration is used by an instruction in the next iteration, we have modified the number of stall cycle that need to be inserted.

---

Full diff: https://github.com/llvm/llvm-project/pull/99451.diff

2 Files Affected:

• (modified) llvm/lib/CodeGen/WindowScheduler.cpp (+5-3)
• (added) llvm/test/CodeGen/Hexagon/swp-ws-stall-cycle.mir (+59)

diff --git a/llvm/lib/CodeGen/WindowScheduler.cpp b/llvm/lib/CodeGen/WindowScheduler.cpp
index 0777480499e55..fcadc18ac8ef8 100644
--- a/llvm/lib/CodeGen/WindowScheduler.cpp
+++ b/llvm/lib/CodeGen/WindowScheduler.cpp
@@ -485,6 +485,7 @@ int WindowScheduler::calculateMaxCycle(ScheduleDAGInstrs &DAG,
 // ========================================
 int WindowScheduler::calculateStallCycle(unsigned Offset, int MaxCycle) {
   int MaxStallCycle = 0;
+  int CurrentII = MaxCycle + 1;
   auto Range = getScheduleRange(Offset, SchedInstrNum);
   for (auto &MI : Range) {
     auto *SU = TripleDAG->getSUnit(&MI);
@@ -492,8 +493,9 @@ int WindowScheduler::calculateStallCycle(unsigned Offset, int MaxCycle) {
     for (auto &Succ : SU->Succs) {
       if (Succ.isWeak() || Succ.getSUnit() == &TripleDAG->ExitSU)
         continue;
-      // If the expected cycle does not exceed MaxCycle, no check is needed.
-      if (DefCycle + (int)Succ.getLatency() <= MaxCycle)
+      // If the expected cycle does not exceed loop initiation interval, no
+      // check is needed.
+      if (DefCycle + (int)Succ.getLatency() <= CurrentII)
         continue;
       // If the cycle of the scheduled MI A is less than that of the scheduled
       // MI B, the scheduling will fail because the lifetime of the
@@ -503,7 +505,7 @@ int WindowScheduler::calculateStallCycle(unsigned Offset, int MaxCycle) {
       if (DefCycle < UseCycle)
         return WindowIILimit;
       // Get the stall cycle introduced by the register between two trips.
-      int StallCycle = DefCycle + (int)Succ.getLatency() - MaxCycle - UseCycle;
+      int StallCycle = DefCycle + (int)Succ.getLatency() - CurrentII - UseCycle;
       MaxStallCycle = std::max(MaxStallCycle, StallCycle);
     }
   }
diff --git a/llvm/test/CodeGen/Hexagon/swp-ws-stall-cycle.mir b/llvm/test/CodeGen/Hexagon/swp-ws-stall-cycle.mir
new file mode 100644
index 0000000000000..ddba67d78eb58
--- /dev/null
+++ b/llvm/test/CodeGen/Hexagon/swp-ws-stall-cycle.mir
@@ -0,0 +1,59 @@
+# REQUIRES: asserts
+# RUN: llc --march=hexagon %s -run-pass=pipeliner -debug-only=pipeliner \
+# RUN: -window-sched=force -filetype=null -verify-machineinstrs \
+# RUN: -window-region-limit=1 -window-search-ratio=100 -window-diff-limit=0 \
+# RUN: 2>&1 | FileCheck %s
+
+# CHECK-LABEL: Start analyzing II
+# CHECK: MaxStallCycle is 0
+# CHECK-LABEL: Start analyzing II
+# CHECK: MaxStallCycle is 0
+# CHECK-LABEL: Start analyzing II
+# CHECK: MaxStallCycle is 0
+
+---
+name:            test_window_stall_cycle
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    successors: %bb.3(0x40000000), %bb.1(0x40000000)
+    liveins: $r0, $r1
+  
+    %0:intregs = COPY $r1
+    %1:intregs = COPY $r0
+    %2:intregs = nsw A2_add %0, %1
+    %3:intregs = S2_lsr_i_r_acc %2, %2, 31
+    %4:intregs = S2_asr_i_r killed %3, 1
+    %5:predregs = C2_cmpgt %1, %4
+    %6:intregs = A2_tfrsi 0
+    J2_jumpt killed %5, %bb.3, implicit-def dead $pc
+    J2_jump %bb.1, implicit-def dead $pc
+  
+  bb.1:
+    successors: %bb.2(0x80000000)
+  
+    %7:intregs = A2_addi %4, 2
+    %8:intregs = A2_tfrsi 0
+    %9:intregs = A2_sub %4, %1
+    %10:intregs = A2_addi %9, 1
+    %11:intregs = COPY %10
+    J2_loop0r %bb.2, %11, implicit-def $lc0, implicit-def $sa0, implicit-def $usr
+  
+  bb.2 (machine-block-address-taken):
+    successors: %bb.3(0x04000000), %bb.2(0x7c000000)
+  
+    %12:intregs = PHI %7, %bb.1, %13, %bb.2
+    %14:intregs = PHI %8, %bb.1, %15, %bb.2
+    %16:intregs = PHI %8, %bb.1, %17, %bb.2
+    %18:intregs, %13:intregs = L2_loadri_pi %12, -4
+    %17:intregs = nsw A2_add killed %18, %16
+    %15:intregs = A2_max %17, %14
+    ENDLOOP0 %bb.2, implicit-def $pc, implicit-def $lc0, implicit $sa0, implicit $lc0
+    J2_jump %bb.3, implicit-def dead $pc
+  
+  bb.3:
+    %19:intregs = PHI %6, %bb.0, %15, %bb.2
+    $r0 = COPY %19
+    PS_jmpret $r31, implicit-def dead $pc, implicit $r0
+
+...

[huaatian]

Please provide a schematic diagram for the calculation of stall cycles.

[kaiyan96]

To make the changes in this PR much clear, here is an example of StallCycle calculation.

Without adding 1 to MaxCycle, StallCycle would be 4 instead of 3, which is not correct.

[huaatian]

LGTM

[huaatian]

-> "does not exceed CurrentII"

[kaiyan96]

Updated.

[huaatian]

I think the calculations here are correct. Do you see any other changes needed for this PR?@arsenm

[AreaZR]

Does this affect codegen correctness or is this just resulting in suboptimal codegen? If it is the former, should this be backported to 19.x?

[huaatian]

Does this affect codegen correctness or is this just resulting in suboptimal codegen? If it is the former, should this be backported to 19.x?

This is just resulting in suboptimal codegen. Since all stall cycles increase by 1, there is relatively no change.