[PATCH] D79590: [lld] Add a new output section ".text.unknown" for funtions with unknown hotness info especially in sampleFDO

[Wei Mi via Phabricator via llvm-commits]

wmi updated this revision to Diff 262732.
wmi added a comment.

Address MaskRay's comment.


Repository:
  rLLD LLVM Linker

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79590/new/

https://reviews.llvm.org/D79590

Files:
  lld/ELF/Writer.cpp
  lld/test/ELF/text-section-prefix.s


Index: lld/test/ELF/text-section-prefix.s
===================================================================
--- lld/test/ELF/text-section-prefix.s
+++ lld/test/ELF/text-section-prefix.s
@@ -1,6 +1,7 @@
 # REQUIRES: x86
 ## -z keep-text-section-prefix separates text sections with prefix .text.hot,
-## .text.unlikely, .text.startup, or .text.exit, in the absence of a SECTIONS command.
+## .text.unknown, .text.unlikely, .text.startup, or .text.exit, in the absence
+## of a SECTIONS command.
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
 # RUN: ld.lld %t.o -o %t1
@@ -13,9 +14,10 @@
 
 # KEEP:      [ 1] .text
 # KEEP-NEXT: [ 2] .text.hot
-# KEEP-NEXT: [ 3] .text.startup
-# KEEP-NEXT: [ 4] .text.exit
-# KEEP-NEXT: [ 5] .text.unlikely
+# KEEP-NEXT: [ 3] .text.unknown
+# KEEP-NEXT: [ 4] .text.startup
+# KEEP-NEXT: [ 5] .text.exit
+# KEEP-NEXT: [ 6] .text.unlikely
 
 # NOKEEP:    [ 1] .text
 # NOKEEP-NOT:     .text
@@ -29,6 +31,7 @@
 # SCRIPT:      .text
 # SCRIPT-NEXT: .text.f
 # SCRIPT-NEXT: .text.hot.f_hot
+# SCRIPT-NEXT: .text.unknown.f_unknown
 # SCRIPT-NEXT: .text.startup.f_startup
 # SCRIPT-NEXT: .text.exit.f_exit
 # SCRIPT-NEXT: .text.unlikely.f_unlikely
@@ -43,6 +46,9 @@
 .section .text.hot.f_hot,"ax"
   nop
 
+.section .text.unknown.f_unknown,"ax"
+  nop
+
 .section .text.startup.f_startup,"ax"
   nop
 
Index: lld/ELF/Writer.cpp
===================================================================
--- lld/ELF/Writer.cpp
+++ lld/ELF/Writer.cpp
@@ -122,13 +122,18 @@
   // When no SECTIONS is specified, emulate GNU ld's internal linker scripts
   // by grouping sections with certain prefixes.
 
-  // GNU ld places text sections with prefix ".text.hot.", ".text.unlikely.",
-  // ".text.startup." or ".text.exit." before others. We provide an option -z
-  // keep-text-section-prefix to group such sections into separate output
-  // sections. This is more flexible. See also sortISDBySectionOrder().
+  // GNU ld places text sections with prefix ".text.hot.", ".text.unknown.",
+  // ".text.unlikely.", ".text.startup." or ".text.exit." before others.
+  // We provide an option -z keep-text-section-prefix to group such sections
+  // into separate output sections. This is more flexible. See also
+  // sortISDBySectionOrder().
+  // ".text.unknown" means the hotness of the section is unknown. When
+  // SampleFDO is used, if a function doesn't have sample, it could be very
+  // cold or it could be a new function never being sampled. Those functions
+  // will be kept in the ".text.unknown" section.
   if (config->zKeepTextSectionPrefix)
-    for (StringRef v :
-         {".text.hot.", ".text.unlikely.", ".text.startup.", ".text.exit."})
+    for (StringRef v : {".text.hot.", ".text.unknown.", ".text.unlikely.",
+                        ".text.startup.", ".text.exit."})
       if (isSectionPrefix(v, s->name))
         return v.drop_back();
 


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