[TableGen] Add support for per-write cycle tunables

[rj-jesus]

This patch adds support for describing per-write resource cycle counts
for ReadAdvance records via a new optional field called tunables.

This makes it possible to declare ReadAdvance records such as:

  def : ReadAdvance<Read_C, 1, [Write_A, Write_B], [2]>;

The above will effectively declare two entries in the ReadAdvance
table for Read_C, one for Write_A with a cycle count of 1+2, and one for
Write_B with a cycle count of 1+0 (omitted values are assumed 0).

The field tunables provides a list of deltas relative to the base
cycle count of the ReadAdvance. Since the field is optional and
defaults to a list of 0's, this change doesn't affect current targets.

[llvmbot]

@llvm/pr-subscribers-tablegen

Author: Ricardo Jesus (rj-jesus)

Changes

This patch adds support for describing per-write resource cycle counts
for ReadAdvance records via a new optional field called tunables.

This makes it possible to declare ReadAdvance records such as:

  def : ReadAdvance<Read_C, 1, [Write_A, Write_B], [2]>;

The above will effectively declare two entries in the ReadAdvance
table for Read_C, one for Write_A with a cycle count of 1+2, and one for
Write_B with a cycle count of 1+0 (omitted values are assumed 0).

The field tunables provides a list of deltas relative to the base
cycle count of the ReadAdvance. Since the field is optional and
defaults to a list of 0's, this change doesn't affect current targets.

---

Full diff: https://github.com/llvm/llvm-project/pull/125870.diff

3 Files Affected:

• (modified) llvm/include/llvm/Target/TargetSchedule.td (+9-5)
• (added) llvm/test/TableGen/PreWriteCycleCount.td (+41)
• (modified) llvm/utils/TableGen/SubtargetEmitter.cpp (+13-6)

diff --git a/llvm/include/llvm/Target/TargetSchedule.td b/llvm/include/llvm/Target/TargetSchedule.td
index 2562ed0901303fe..4b82c1e2410fc8c 100644
--- a/llvm/include/llvm/Target/TargetSchedule.td
+++ b/llvm/include/llvm/Target/TargetSchedule.td
@@ -321,9 +321,11 @@ class SchedWriteRes<list<ProcResourceKind> resources> : SchedWrite,
 // Define values common to ReadAdvance and SchedReadAdvance.
 //
 // SchedModel ties these resources to a processor.
-class ProcReadAdvance<int cycles, list<SchedWrite> writes = []> {
+class ProcReadAdvance<int cycles, list<SchedWrite> writes = [],
+                      list<int> tunables = []> {
   int Cycles = cycles;
   list<SchedWrite> ValidWrites = writes;
+  list<int> CycleTunables = tunables;
   // Allow a processor to mark some scheduling classes as unsupported
   // for stronger verification.
   bit Unsupported = false;
@@ -340,15 +342,17 @@ class ProcReadAdvance<int cycles, list<SchedWrite> writes = []> {
 // indicate operands that are always read this number of Cycles later
 // than a normal register read, allowing the read's parent instruction
 // to issue earlier relative to the writer.
-class ReadAdvance<SchedRead read, int cycles, list<SchedWrite> writes = []>
-  : ProcReadAdvance<cycles, writes> {
+class ReadAdvance<SchedRead read, int cycles, list<SchedWrite> writes = [],
+                  list<int> tunables = []>
+  : ProcReadAdvance<cycles, writes, tunables> {
   SchedRead ReadType = read;
 }
 
 // Directly associate a new SchedRead type with a delay and optional
 // pipeline bypass. For use with InstRW or ItinRW.
-class SchedReadAdvance<int cycles, list<SchedWrite> writes = []> : SchedRead,
-  ProcReadAdvance<cycles, writes>;
+class SchedReadAdvance<int cycles, list<SchedWrite> writes = [],
+                       list<int> tunables = []>
+  : SchedRead, ProcReadAdvance<cycles, writes, tunables>;
 
 // Define SchedRead defaults. Reads seldom need special treatment.
 def ReadDefault : SchedRead;
diff --git a/llvm/test/TableGen/PreWriteCycleCount.td b/llvm/test/TableGen/PreWriteCycleCount.td
new file mode 100644
index 000000000000000..40260a9bacef449
--- /dev/null
+++ b/llvm/test/TableGen/PreWriteCycleCount.td
@@ -0,0 +1,41 @@
+// RUN: llvm-tblgen -gen-subtarget -I %p/../../include %s 2>&1 | FileCheck %s
+
+// Make sure that ReadAdvance entries with multiple writes are correctly
+// handled.
+
+include "llvm/Target/Target.td"
+
+def MyTarget : Target;
+
+let OutOperandList = (outs), InOperandList = (ins) in {
+  def Inst_A : Instruction;
+  def Inst_B : Instruction;
+  def Inst_C : Instruction;
+}
+
+let CompleteModel = 0 in {
+  def SchedModel_A: SchedMachineModel;
+}
+
+def Read_D : SchedRead;
+
+// CHECK: extern const llvm::MCReadAdvanceEntry MyTargetReadAdvanceTable[] = {
+// CHECK-NEXT:  {0,  0,  0}, // Invalid
+// CHECK-NEXT:  {0,  1,  1}, // #1
+// CHECK-NEXT:  {0,  2,  3}, // #2
+// CHECK-NEXT:  {0,  3,  2} // #3
+// CHECK-NEXT: }; // MyTargetReadAdvanceTable
+
+let SchedModel = SchedModel_A in {
+  def Write_A : SchedWriteRes<[]>;
+  def Write_B : SchedWriteRes<[]>;
+  def Write_C : SchedWriteRes<[]>;
+
+  def : InstRW<[Write_A], (instrs Inst_A)>;
+  def : InstRW<[Write_B], (instrs Inst_B)>;
+  def : InstRW<[Write_C, Read_D], (instrs Inst_C)>;
+
+  def : ReadAdvance<Read_D, 2, [Write_A, Write_B, Write_C], [-1, 1]>;
+}
+
+def ProcessorA: ProcessorModel<"ProcessorA", SchedModel_A, []>;
diff --git a/llvm/utils/TableGen/SubtargetEmitter.cpp b/llvm/utils/TableGen/SubtargetEmitter.cpp
index 49362ff5ef65563..ec0990292738230 100644
--- a/llvm/utils/TableGen/SubtargetEmitter.cpp
+++ b/llvm/utils/TableGen/SubtargetEmitter.cpp
@@ -1308,23 +1308,30 @@ void SubtargetEmitter::genSchedClassTables(const CodeGenProcModel &ProcModel,
       }
       ConstRecVec ValidWrites =
           ReadAdvance->getValueAsListOfDefs("ValidWrites");
-      IdxVec WriteIDs;
+      std::vector<int64_t> CycleTunables =
+          ReadAdvance->getValueAsListOfInts("CycleTunables");
+      std::vector<std::pair<unsigned, int>> WriteIDs;
+      if (!CycleTunables.empty() && CycleTunables.size() > ValidWrites.size())
+        PrintFatalError(ReadAdvance->getLoc(),
+                        "If specified, CycleTunables must have at most the "
+                        "same number of elements of ValidWrites.\n");
+      CycleTunables.resize(ValidWrites.size(), 0);
       if (ValidWrites.empty())
-        WriteIDs.push_back(0);
+        WriteIDs.push_back(std::make_pair(0, 0));
       else {
-        for (const Record *VW : ValidWrites) {
+        for (const auto [VW, CT] : zip_equal(ValidWrites, CycleTunables)) {
           unsigned WriteID = SchedModels.getSchedRWIdx(VW, /*IsRead=*/false);
           assert(WriteID != 0 &&
                  "Expected a valid SchedRW in the list of ValidWrites");
-          WriteIDs.push_back(WriteID);
+          WriteIDs.push_back(std::make_pair(WriteID, CT));
         }
       }
       llvm::sort(WriteIDs);
-      for (unsigned W : WriteIDs) {
+      for (const auto &[W, T] : WriteIDs) {
         MCReadAdvanceEntry RAEntry;
         RAEntry.UseIdx = UseIdx;
         RAEntry.WriteResourceID = W;
-        RAEntry.Cycles = ReadAdvance->getValueAsInt("Cycles");
+        RAEntry.Cycles = ReadAdvance->getValueAsInt("Cycles") + T;
         ReadAdvanceEntries.push_back(RAEntry);
       }
     }

[rj-jesus]

Ping. :)

[jurahul]

Also, once that is done, I think you need to re-request the review by clicking the small circle arrow button next to the reviewers, otherwise I am suspecting it won't get back on their radar.

[jurahul]

LGTM, but please wait for @topperc 's (or someone else's) approval as well

[topperc]

LGTM

[rj-jesus]

Thanks very much for the comments!