cpu-o3: Refine reverse-tick stall attribution

Change-Id: I4dff1631acaf7a07f663f05c878d7bd40cc9740a

Author: jensen-yan

docs/tools/topdown/reverseTickRootCauseTopdown.md

@@ -179,7 +179,31 @@ reverse tick 下，`StallSignals` 已经承担同拍背压的传播职责。
 
 ---
 
-## 8. 最后总结
+## 8. 新增的结构性背压原因
+
+在这次 reverse tick 统计修复之后，原本容易被吞到 `OtherStall` 里的两类结构性背压，被显式拆成了独立原因：
+
+### `RegFull`
+
+- 含义：rename 本级因为物理寄存器资源不足，无法继续向前推进
+- 位置：根因首先发生在 rename
+- 传播：如果这个原因继续向前反压，则 `rename/decode/fetch StallReason` 都可以看到 `RegFull`
+
+它不属于“未知 stall”，而是明确的 rename 资源瓶颈。
+
+### `ROBFull`
+
+- 含义：commit 侧因为 ROB 剩余空间不足，无法继续吸收来自 rename 的输入
+- 位置：根因首先体现为 commit 对上游的容量背压
+- 传播：如果这个原因沿着背压链向前扩散，则 `rename/decode/fetch StallReason` 都可以看到 `ROBFull`
+
+这里的 `ROBFull` 更接近“ROB 容量背压”，而不是“ROB head 对应指令的执行根因”。
+
+因此，这两个 reason 的引入，主要是为了把原本模糊的 `OtherStall` 拆解成更有解释力的结构性原因，而不是改变一级 topdown 行为。
+
+---
+
+## 9. 最后总结
 
 推荐的理解方式是：
 

src/cpu/o3/comm.hh

@@ -101,6 +101,8 @@ enum StallReason {
     ScalarReadyButNotIssued,  // B
     ResumeUnblock,  // B
     CommitSquash,  // BS
+    ROBFull,  // B
+    RegFull,  // B
     OtherStall,  // B
     NumStallReasons
 };

src/cpu/o3/commit.cc

@@ -1863,7 +1863,7 @@ Commit::moveInstsToBuffer()
         } else if (block) {
             block_reason = robInfoFromIEW->iewInfo[i].robHeadStallReason;
             if (block_reason == StallReason::NoStall) {
-                block_reason = StallReason::OtherStall;
+                block_reason = StallReason::ROBFull;
             }
         }
         DPRINTF(Commit, "Thread %i: block %i robblock %i active %i\n", i, block, robblock, active);

src/cpu/o3/fetch.cc

@@ -1221,6 +1221,7 @@ Fetch::initializeTickState()
     bool status_change = false;
 
     wroteToTimeBuffer = false;
+    setAllFetchStalls(StallReason::NoStall);
 
     // get the distribution of fetch status
     fetchStats.fetchStatusDist[fetchStatus[0]]++;
@@ -1304,8 +1305,21 @@ Fetch::sendInstructionsToDecode()
     }
     if (!any_thread_active) {
         // All threads are blocked, no instructions to send
+        ThreadID blocked_tid = InvalidThreadID;
+        for (int i = 0; i < numThreads; i++) {
+            if (stallSig->blockFetch[i]) {
+                blocked_tid = i;
+                break;
+            }
+        }
+
+        if (blocked_tid != InvalidThreadID) {
+            setAllFetchStalls(stallSig->fetchBlockReason[blocked_tid]);
+        }
+
+        toDecode->fetchStallReason = stallReason;
+
         for (int i = 0; i < numThreads; i++) {
-            updateStallReasons(0, i);
             measureFrontendBubbles(0, i);
         }
         return;
@@ -1350,7 +1364,9 @@ Fetch::sendInstructionsToDecode()
 void
 Fetch::updateStallReasons(unsigned insts_to_decode, ThreadID tid)
 {
-    if (insts_to_decode == 0) {
+    if (stallSig->blockFetch[tid]) {
+        setAllFetchStalls(stallSig->fetchBlockReason[tid]);
+    } else if (insts_to_decode == 0) {
         // fetch stalled
         if (stallReason[0] != StallReason::NoStall) {
             // previously set stall reason

src/cpu/o3/iew.cc

@@ -303,6 +303,8 @@ IEW::IEWStats::IEWStats(CPU *cpu)
         {StallReason::Atomic,"Atomic"},
         {StallReason::ResumeUnblock, "ResumeUnblock"},
         {StallReason::CommitSquash, "CommitSquash"},
+        {StallReason::ROBFull, "ROBFull"},
+        {StallReason::RegFull, "RegFull"},
         {StallReason::OtherStall, "OtherStall"},
         {StallReason::OtherFetchStall, "OtherFetchStall"},
         {StallReason::FTQBubble, "FTQBubble"},

src/cpu/o3/rename.cc

@@ -354,7 +354,9 @@ Rename::tick()
         } else if (!can_rename) {
             block_reason = checkRenameStallFromIEW(i);
             if (block_reason == StallReason::NoStall) {
-                block_reason = StallReason::OtherStall;
+                block_reason = StallReason::RegFull;
+                ++stats.fullRegistersEvents;
+                stats.stallEvents[RegFull]++;
             }
         }
         DPRINTF(Rename, "[tid:%i] blockRename: %i, canRename: %i, block: %i, active: %i\n",
@@ -562,7 +564,9 @@ Rename::renameInsts(ThreadID tid)
         if (breakRename == StallReason::NoStall) {
             breakRename = checkRenameStallFromIEW(tid);
             if (breakRename == StallReason::NoStall) {
-                breakRename = StallReason::OtherStall;
+                breakRename = StallReason::RegFull;
+                ++stats.fullRegistersEvents;
+                stats.stallEvents[RegFull]++;
             }
         }
         blockReason = breakRename;
@@ -993,19 +997,21 @@ StallReason
 Rename::checkRenameStallFromIEW(ThreadID tid)
 {
     StallReason robHeadStallReason = fromIEW->iewInfo[tid].robHeadStallReason;
-    return robHeadStallReason;
-
-    // if (robHeadStallReason == StallReason::NoStall) {
-    //     if (calcFreeLQEntries(tid) <= 0) {
-    //         return fromIEW->iewInfo[tid].lqHeadStallReason;
-    //     } else if (calcFreeSQEntries(tid) <= 0) {
-    //         return fromIEW->iewInfo[tid].sqHeadStallReason;
-    //     } else {
-    //         return robHeadStallReason;
-    //     }
-    // } else {
-    //     return robHeadStallReason;
-    // }
+    if (robHeadStallReason != StallReason::NoStall) {
+        return robHeadStallReason;
+    }
+
+    StallReason lqHeadStallReason = fromIEW->iewInfo[tid].lqHeadStallReason;
+    if (lqHeadStallReason != StallReason::NoStall) {
+        return lqHeadStallReason;
+    }
+
+    StallReason sqHeadStallReason = fromIEW->iewInfo[tid].sqHeadStallReason;
+    if (sqHeadStallReason != StallReason::NoStall) {
+        return sqHeadStallReason;
+    }
+
+    return StallReason::NoStall;
 }
 
 } // namespace o3