[llvm] [BOLT] Refactor NewTextSegmentAddress handling (PR #145950)

[Maksim Panchenko via llvm-commits]

https://github.com/maksfb created https://github.com/llvm/llvm-project/pull/145950

Refactor the code for NewTextSegmentAddress to correctly point at the true start of the segment when PHDR table is placed at the beginning. We used to offset NewTextSegmentAddress by PHDR table plus cache line alignment.

NFC for proper binaries. Some YAML binaries from our tests will diverge due to bad segment address/offset alignment.

>From 0ed9572d6306bf95d349590196b55bc48dec469b Mon Sep 17 00:00:00 2001
From: Maksim Panchenko <maks at fb.com>
Date: Tue, 24 Jun 2025 23:18:07 -0700
Subject: [PATCH] [BOLT] Refactor NewTextSegmentAddress handling

Refactor the code for NewTextSegmentAddress to correctly point at the
true start of the segment when PHDR table is placed at the beginning.
We used to offset NewTextSegmentAddress by PHDR table plus cache line
alignment.

NFC for proper binaries. Some YAML binaries from our tests will diverge
due to bad segment address/offset alignment.
---
 bolt/lib/Rewrite/RewriteInstance.cpp | 40 +++++++++++-----------------
 1 file changed, 15 insertions(+), 25 deletions(-)

diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index 93bd93b6cb984..dc7591a1d1226 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -626,6 +626,9 @@ Error RewriteInstance::discoverStorage() {
     NextAvailableAddress += BC->PageAlign;
   }
 
+  NewTextSegmentAddress = NextAvailableAddress;
+  NewTextSegmentOffset = NextAvailableOffset;
+
   if (!opts::UseGnuStack && !BC->IsLinuxKernel) {
     // This is where the black magic happens. Creating PHDR table in a segment
     // other than that containing ELF header is tricky. Some loaders and/or
@@ -652,6 +655,8 @@ Error RewriteInstance::discoverStorage() {
 
     PHDRTableAddress = NextAvailableAddress;
     PHDRTableOffset = NextAvailableOffset;
+    NewTextSegmentAddress = NextAvailableAddress;
+    NewTextSegmentOffset = NextAvailableOffset;
 
     // Reserve space for 3 extra pheaders.
     unsigned Phnum = Obj.getHeader().e_phnum;
@@ -664,14 +669,12 @@ Error RewriteInstance::discoverStorage() {
 
     NextAvailableAddress += Phnum * sizeof(ELF64LEPhdrTy);
     NextAvailableOffset += Phnum * sizeof(ELF64LEPhdrTy);
-  }
 
-  // Align at cache line.
-  NextAvailableAddress = alignTo(NextAvailableAddress, 64);
-  NextAvailableOffset = alignTo(NextAvailableOffset, 64);
+    // Align at cache line.
+    NextAvailableAddress = alignTo(NextAvailableAddress, 64);
+    NextAvailableOffset = alignTo(NextAvailableOffset, 64);
+  }
 
-  NewTextSegmentAddress = NextAvailableAddress;
-  NewTextSegmentOffset = NextAvailableOffset;
   BC->LayoutStartAddress = NextAvailableAddress;
 
   // Tools such as objcopy can strip section contents but leave header
@@ -4133,13 +4136,8 @@ void RewriteInstance::mapAllocatableSections(
     }
 
     if (SType == ST_READONLY) {
-      if (PHDRTableAddress) {
-        // Segment size includes the size of the PHDR area.
-        NewTextSegmentSize = NextAvailableAddress - PHDRTableAddress;
-      } else if (NewTextSegmentAddress) {
-        // Existing PHDR table would be updated.
+      if (NewTextSegmentAddress)
         NewTextSegmentSize = NextAvailableAddress - NewTextSegmentAddress;
-      }
     } else if (SType == ST_READWRITE) {
       NewWritableSegmentSize = NextAvailableAddress - NewWritableSegmentAddress;
       // Restore NextAvailableAddress if no new writable sections
@@ -4186,9 +4184,7 @@ void RewriteInstance::patchELFPHDRTable() {
   // NOTE Currently .eh_frame_hdr appends to the last segment, recalculate
   // last segments size based on the NextAvailableAddress variable.
   if (!NewWritableSegmentSize) {
-    if (PHDRTableAddress)
-      NewTextSegmentSize = NextAvailableAddress - PHDRTableAddress;
-    else if (NewTextSegmentAddress)
+    if (NewTextSegmentAddress)
       NewTextSegmentSize = NextAvailableAddress - NewTextSegmentAddress;
   } else {
     NewWritableSegmentSize = NextAvailableAddress - NewWritableSegmentAddress;
@@ -4201,15 +4197,9 @@ void RewriteInstance::patchELFPHDRTable() {
     SmallVector<ELF64LEPhdrTy, 3> NewPhdrs;
     ELF64LEPhdrTy NewPhdr;
     NewPhdr.p_type = ELF::PT_LOAD;
-    if (PHDRTableAddress) {
-      NewPhdr.p_offset = PHDRTableOffset;
-      NewPhdr.p_vaddr = PHDRTableAddress;
-      NewPhdr.p_paddr = PHDRTableAddress;
-    } else {
-      NewPhdr.p_offset = NewTextSegmentOffset;
-      NewPhdr.p_vaddr = NewTextSegmentAddress;
-      NewPhdr.p_paddr = NewTextSegmentAddress;
-    }
+    NewPhdr.p_offset = NewTextSegmentOffset;
+    NewPhdr.p_vaddr = NewTextSegmentAddress;
+    NewPhdr.p_paddr = NewTextSegmentAddress;
     NewPhdr.p_filesz = NewTextSegmentSize;
     NewPhdr.p_memsz = NewTextSegmentSize;
     NewPhdr.p_flags = ELF::PF_X | ELF::PF_R;
@@ -4270,7 +4260,7 @@ void RewriteInstance::patchELFPHDRTable() {
   };
 
   auto writeNewSegmentPhdrs = [&]() {
-    if (PHDRTableAddress || NewTextSegmentSize) {
+    if (NewTextSegmentSize) {
       SmallVector<ELF64LE::Phdr, 3> NewPhdrs = createNewPhdrs();
       OS.write(reinterpret_cast<const char *>(NewPhdrs.data()),
                sizeof(ELF64LE::Phdr) * NewPhdrs.size());