[llvm] [llvm-profgen] Preserve zero-valued first PT_LOAD address (PR #212258)

[Sergey Shcherbinin via llvm-commits]

https://github.com/SergeyShch01 created https://github.com/llvm/llvm-project/pull/212258

FirstLoadableAddress was initialized with if (!FirstLoadableAddress), so a valid first PT_LOAD at vaddr 0 was treated as unset and overwritten by a later segment. Track whether the first loadable segment has been seen instead.

Assisted by GPT-5

>From 49a2cee58f26c064241c3e6dd8d137036fa14c0c Mon Sep 17 00:00:00 2001
From: Sergey Shcherbinin <sscherbinin at nvidia.com>
Date: Mon, 27 Jul 2026 18:04:33 +0400
Subject: [PATCH] [llvm-profgen] Preserve zero-valued first PT_LOAD address

FirstLoadableAddress was initialized with if (!FirstLoadableAddress), so a
valid first PT_LOAD at vaddr 0 was treated as unset and overwritten by a later
segment. Track whether the first loadable segment has been seen instead.

Assisted by GPT-5
---
 .../X86/first-loadable-address.test           | 59 +++++++++++++++++++
 llvm/tools/llvm-profgen/ProfiledBinary.cpp    |  5 +-
 2 files changed, 63 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/tools/llvm-profgen/X86/first-loadable-address.test

diff --git a/llvm/test/tools/llvm-profgen/X86/first-loadable-address.test b/llvm/test/tools/llvm-profgen/X86/first-loadable-address.test
new file mode 100644
index 0000000000000..50f9f5dcc2f63
--- /dev/null
+++ b/llvm/test/tools/llvm-profgen/X86/first-loadable-address.test
@@ -0,0 +1,59 @@
+# RUN: split-file %s %t
+# RUN: yaml2obj %t/binary.yaml -o %t/first-loadable-address.exe
+# RUN: llvm-profgen --binary=%t/first-loadable-address.exe \
+# RUN:   --perfscript=%t/perfscript --skip-symbolization --format=text \
+# RUN:   --use-offset=1 --use-first-loadable-segment-as-base=1 \
+# RUN:   --output=%t/profile
+# RUN: FileCheck %s --input-file=%t/profile
+
+## The first PT_LOAD has a valid zero virtual address. Ensure it is not treated
+## as an unset value and overwritten by the following executable PT_LOAD.
+## With the default 4K page size, the text segment file offset is 0x1000, so the
+## mmap event uses that offset rather than zero.
+# CHECK:      0
+# CHECK-NEXT: 1
+# CHECK-NEXT: 11008->11000:1
+
+#--- binary.yaml
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_X86_64
+  Entry:   0x0000000000011000
+Sections:
+  - Name:         .text
+    Type:         SHT_PROGBITS
+    Flags:        [ SHF_ALLOC, SHF_EXECINSTR ]
+    Address:      0x0000000000011000
+    Offset:       0x0000000000001000
+    AddressAlign: 0x1
+    ## 11000: nop*8
+    ## 11008: ret
+    Content:      9090909090909090C3
+ProgramHeaders:
+  - Type:     PT_LOAD
+    Flags:    [ PF_R ]
+    Offset:   0x0
+    VAddr:    0x0
+    FileSize: 0x1000
+    MemSize:  0x1000
+    Align:    0x1000
+  - Type:     PT_LOAD
+    Flags:    [ PF_X, PF_R ]
+    VAddr:    0x0000000000011000
+    Align:    0x1000
+    FirstSec: .text
+    LastSec:  .text
+Symbols:
+  - Name:    foo
+    Type:    STT_FUNC
+    Section: .text
+    Binding: STB_GLOBAL
+    Value:   0x0000000000011000
+    Size:    0x9
+
+#--- perfscript
+PERF_RECORD_MMAP2 1/1: [0x700000001000(0x1000) @ 0x1000 00:00 0 0]: r-xp /tmp/first-loadable-address.exe
+700000001000 0x700000001008/0x700000001000/P/-/-/0
diff --git a/llvm/tools/llvm-profgen/ProfiledBinary.cpp b/llvm/tools/llvm-profgen/ProfiledBinary.cpp
index 08fd6917c9f3a..2d147d0af7353 100644
--- a/llvm/tools/llvm-profgen/ProfiledBinary.cpp
+++ b/llvm/tools/llvm-profgen/ProfiledBinary.cpp
@@ -367,12 +367,15 @@ void ProfiledBinary::setPreferredTextSegmentAddresses(const ELFFile<ELFT> &Obj,
   // 4K page now. Note that we don't use EXEC_PAGESIZE from <linux/param.h>
   // because we may build the tools on non-linux.
   uint64_t PageSize = 0x1000;
+  bool SeenFirstLoadableSegment = false;
   for (const typename ELFT::Phdr &Phdr : PhdrRange) {
     if (Phdr.p_type == ELF::PT_INTERP)
       HasInterp = true;
     if (Phdr.p_type == ELF::PT_LOAD) {
-      if (!FirstLoadableAddress)
+      if (!SeenFirstLoadableSegment) {
         FirstLoadableAddress = Phdr.p_vaddr & ~(PageSize - 1U);
+        SeenFirstLoadableSegment = true;
+      }
       if (Phdr.p_flags & ELF::PF_X) {
         // Segments will always be loaded at a page boundary.
         PreferredTextSegmentAddresses.push_back(Phdr.p_vaddr &