[PATCH] D77639: [DebugInfo][NFC] Early-exit when analysing for single-location variables

[Jeremy Morse via Phabricator via llvm-commits]

jmorse created this revision.
jmorse added reviewers: aprantl, dstenb, NikolaPrica.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

This is a performance patch that hoists two conditions in DwarfDebug's validThroughout to avoid a linear-scan of all instructions in a block. We now exit early if validThrougout will never return true for the variable location. No test as this is a performance change, but:

- It halves the time in DWARF emission for several code samples I'm working on, which have large volumes of inlining to deal with,
- A clang-10 stage2 build produces an identical binary with/without this patch.

The first added clause filters for the two circumstances where validThroughout will return true. The second added clause should be identical to the one that's deleted from after the linear-scan.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D77639

Files:
  llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp


Index: llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1480,11 +1480,24 @@
   if (LSRange.size() == 0)
     return false;
 
+  // If this range neither open ended nor a constant, then it is not a
+  // candidate for being validThroughout.
+  if (RangeEnd != nullptr && !DbgValue->getOperand(0).isImm())
+    return false;
+
   // Determine if the DBG_VALUE is valid at the beginning of its lexical block.
   const MachineInstr *LScopeBegin = LSRange.front().first;
   // Early exit if the lexical scope begins outside of the current block.
   if (LScopeBegin->getParent() != MBB)
     return false;
+
+  // If there are instructions belonging to our scope in another block, and
+  // we're not a constant (see DWARF2 comment below), then we can't be
+  // validThroughout.
+  const MachineInstr *LScopeEnd = LSRange.back().second;
+  if (RangeEnd && LScopeEnd->getParent() != MBB)
+    return false;
+
   MachineBasicBlock::const_reverse_iterator Pred(DbgValue);
   for (++Pred; Pred != MBB->rend(); ++Pred) {
     if (Pred->getFlag(MachineInstr::FrameSetup))
@@ -1505,11 +1518,6 @@
   if (!RangeEnd)
     return true;
 
-  // Fail if there are instructions belonging to our scope in another block.
-  const MachineInstr *LScopeEnd = LSRange.back().second;
-  if (LScopeEnd->getParent() != MBB)
-    return false;
-
   // Single, constant DBG_VALUEs in the prologue are promoted to be live
   // throughout the function. This is a hack, presumably for DWARF v2 and not
   // necessarily correct. It would be much better to use a dbg.declare instead


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D77639.255637.patch
Type: text/x-patch
Size: 1710 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20200407/641e7a80/attachment.bin>