Dump the "reason" for a compiler temp

Compiler temps are created with a "reason" that is dumped in JitDump.
Save the reason and display it in the local variable table dump.
This is useful when trying to find a particular temp and see what
code has been generated using it.

3 files changed, 25 insertions, 8 deletions

diff --git a/src/jit/compiler.h b/src/jit/compiler.h
index 16e7ebd503..f68642671b 100644
--- a/src/jit/compiler.h
+++ b/src/jit/compiler.h
@@ -850,6 +850,8 @@ public:
 
 #ifdef DEBUG
 public:
+    const char* lvReason;
+
     void PrintVarReg() const
     {
         printf("%s", getRegName(lvRegNum));
diff --git a/src/jit/compiler.hpp b/src/jit/compiler.hpp
index 823cc99038..b4de24ae38 100644
--- a/src/jit/compiler.hpp
+++ b/src/jit/compiler.hpp
@@ -1701,6 +1701,8 @@ inline unsigned Compiler::lvaGrabTemp(bool shortLifetime DEBUGARG(const char* re
     }
 
 #ifdef DEBUG
+    lvaTable[tempNum].lvReason = reason;
+
     if (verbose)
     {
         printf("\nlvaGrabTemp returning %d (", tempNum);
diff --git a/src/jit/lclvars.cpp b/src/jit/lclvars.cpp
index 645f3fe280..0f68c365e2 100644
--- a/src/jit/lclvars.cpp
+++ b/src/jit/lclvars.cpp
@@ -2116,17 +2116,21 @@ void Compiler::StructPromotionHelper::PromoteStructVar(unsigned lclNum)
 // Now grab the temp for the field local.
 
 #ifdef DEBUG
-        char  buf[200];
-        char* bufp = &buf[0];
-
-        sprintf_s(bufp, sizeof(buf), "%s V%02u.%s (fldOffset=0x%x)", "field", lclNum,
+        char buf[200];
+        sprintf_s(buf, sizeof(buf), "%s V%02u.%s (fldOffset=0x%x)", "field", lclNum,
                   compiler->eeGetFieldName(pFieldInfo->fldHnd), pFieldInfo->fldOffset);
 
+        // We need to copy 'buf' as lvaGrabTemp() below caches a copy to its argument.
+        size_t len  = strlen(buf) + 1;
+        char*  bufp = compiler->getAllocator(CMK_DebugOnly).allocate<char>(len);
+        strcpy_s(bufp, len, buf);
+
         if (index > 0)
         {
             noway_assert(pFieldInfo->fldOffset > (pFieldInfo - 1)->fldOffset);
         }
 #endif
+
         // Lifetime of field locals might span multiple BBs, so they must be long lifetime temps.
         unsigned varNum = compiler->lvaGrabTemp(false DEBUGARG(bufp));
 
@@ -2227,12 +2231,16 @@ void Compiler::lvaPromoteLongVars()
             CLANG_FORMAT_COMMENT_ANCHOR;
 
 #ifdef DEBUG
-            char  buf[200];
-            char* bufp = &buf[0];
-
-            sprintf_s(bufp, sizeof(buf), "%s V%02u.%s (fldOffset=0x%x)", "field", lclNum, index == 0 ? "lo" : "hi",
+            char buf[200];
+            sprintf_s(buf, sizeof(buf), "%s V%02u.%s (fldOffset=0x%x)", "field", lclNum, index == 0 ? "lo" : "hi",
                       index * 4);
+
+            // We need to copy 'buf' as lvaGrabTemp() below caches a copy to its argument.
+            size_t len  = strlen(buf) + 1;
+            char*  bufp = getAllocator(CMK_DebugOnly).allocate<char>(len);
+            strcpy_s(bufp, len, buf);
 #endif
+
             unsigned varNum = lvaGrabTemp(false DEBUGARG(bufp)); // Lifetime of field locals might span multiple BBs, so
                                                                  // they are long lifetime temps.
 
@@ -6964,6 +6972,11 @@ void Compiler::lvaDumpEntry(unsigned lclNum, FrameLayoutState curState, size_t r
         }
     }
 
+    if (varDsc->lvReason != nullptr)
+    {
+        printf(" \"%s\"", varDsc->lvReason);
+    }
+
     printf("\n");
 }