[llvm] [PseudoProbe] Use probe id as the base dwarf discriminator for callsites (PR #65685)

[Hongtao Yu via llvm-commits]

https://github.com/htyu created https://github.com/llvm/llvm-project/pull/65685:

With `-fpseudo-probe-for-profiling`, the dwarf discriminator for a callsite will be overwritten to pseudo probe related information for that callsite. The probe information is encoded in a special format (i.e., with all lowest three digits be one) in order to be distinguished from regular dwarf discriminator. The special encoding format will be decoded to zero by the regular discriminator logic. This means all callsites would have a zero discriminator in both the sample profile and the compiler, for classic AutoFDO. This is inconvenient in that no decent classic AutoFDO can be generated from a pseudo probe build. I'm mitigating the issue by allowing callsite probe id to be used as the base dwarf discriminator for classic AutoFDO, since probe id is also unique and can be used to differentiate callsites on the same source line.





>From f94b7a10bf974a35f28741cbf97a1147bcc26942 Mon Sep 17 00:00:00 2001
From: Hongtao Yu <hoy at fb.com>
Date: Wed, 6 Sep 2023 14:55:55 -0700
Subject: [PATCH] Return probe id for probe discriminator

Summary:

Test Plan:

Reviewers:

Subscribers:

Tasks:

Tags:
---
 llvm/include/llvm/IR/DebugInfoMetadata.h                  | 6 ++++++
 llvm/test/Transforms/SampleProfile/pseudo-probe-inline.ll | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/llvm/include/llvm/IR/DebugInfoMetadata.h b/llvm/include/llvm/IR/DebugInfoMetadata.h
index b9f6d39a7491566..35d03aa6e3bb2a4 100644
--- a/llvm/include/llvm/IR/DebugInfoMetadata.h
+++ b/llvm/include/llvm/IR/DebugInfoMetadata.h
@@ -22,6 +22,7 @@
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Metadata.h"
+#include "llvm/IR/PseudoProbe.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Discriminator.h"
@@ -2075,6 +2076,11 @@ class DILocation : public MDNode {
   static unsigned
   getBaseDiscriminatorFromDiscriminator(unsigned D,
                                         bool IsFSDiscriminator = false) {
+    // Return the probe id instead of zero for a pseudo probe discriminator.
+    // This should help differenciate callsites with same line numbers.
+    if (isPseudoProbeDiscriminator(D))
+      return PseudoProbeDwarfDiscriminator::extractProbeIndex(D);
+
     if (IsFSDiscriminator)
       return getMaskedDiscriminator(D, getBaseDiscriminatorBits());
     return getUnsignedFromPrefixEncoding(D);
diff --git a/llvm/test/Transforms/SampleProfile/pseudo-probe-inline.ll b/llvm/test/Transforms/SampleProfile/pseudo-probe-inline.ll
index 8157bebdc3777be..18cbd857d97bb2c 100644
--- a/llvm/test/Transforms/SampleProfile/pseudo-probe-inline.ll
+++ b/llvm/test/Transforms/SampleProfile/pseudo-probe-inline.ll
@@ -106,6 +106,8 @@ if.end:
 ;YAML-NEXT:    - Line:            '1'
 ;YAML-NEXT:    - String:          ':'
 ;YAML-NEXT:    - Column:          '11'
+;YAML-NEXT:    - String:          .
+;YAML-NEXT:    - Disc:            '2'
 ;YAML-NEXT:    - String:          ';'
 ;YAML-NEXT:  ...
 ;YAML:  --- !Analysis