[llvm] r371501 - [yaml2obj] Set p_align to the maximum sh_addralign of contained sections

[Fangrui Song via llvm-commits]

Author: maskray
Date: Tue Sep 10 02:16:34 2019
New Revision: 371501

URL: http://llvm.org/viewvc/llvm-project?rev=371501&view=rev
Log:
[yaml2obj] Set p_align to the maximum sh_addralign of contained sections

The address difference between two sections in a PT_LOAD is a constant.
Consider a hypothetical case (pagesize can be very small, say, 4).

```
.text     sh_addralign=4
.text.hot sh_addralign=16
```

If we set p_align to 4, the PT_LOAD will be loaded at an address which
is a multiple of 4. The address of .text.hot is guaranteed to be a
multiple of 4, but not necessarily a multiple of 16.

This patch deletes the constraint

  if (SHeader->sh_offset == PHeader.p_offset)

Reviewed By: grimar, jhenderson

Differential Revision: https://reviews.llvm.org/D67260

Modified:
    llvm/trunk/lib/ObjectYAML/ELFEmitter.cpp
    llvm/trunk/test/tools/yaml2obj/program-header-align.yaml

Modified: llvm/trunk/lib/ObjectYAML/ELFEmitter.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/ObjectYAML/ELFEmitter.cpp?rev=371501&r1=371500&r2=371501&view=diff
==============================================================================
--- llvm/trunk/lib/ObjectYAML/ELFEmitter.cpp (original)
+++ llvm/trunk/lib/ObjectYAML/ELFEmitter.cpp Tue Sep 10 02:16:34 2019
@@ -645,16 +645,15 @@ void ELFState<ELFT>::setProgramHeaderLay
     PHeader.p_memsz = YamlPhdr.MemSize ? uint64_t(*YamlPhdr.MemSize)
                                        : MemOffset - PHeader.p_offset;
 
-    // Set the alignment of the segment to be the same as the maximum alignment
-    // of the sections with the same offset so that by default the segment
-    // has a valid and sensible alignment.
     if (YamlPhdr.Align) {
       PHeader.p_align = *YamlPhdr.Align;
     } else {
+      // Set the alignment of the segment to be the maximum alignment of the
+      // sections so that by default the segment has a valid and sensible
+      // alignment.
       PHeader.p_align = 1;
       for (Elf_Shdr *SHeader : Sections)
-        if (SHeader->sh_offset == PHeader.p_offset)
-          PHeader.p_align = std::max(PHeader.p_align, SHeader->sh_addralign);
+        PHeader.p_align = std::max(PHeader.p_align, SHeader->sh_addralign);
     }
   }
 }

Modified: llvm/trunk/test/tools/yaml2obj/program-header-align.yaml
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/tools/yaml2obj/program-header-align.yaml?rev=371501&r1=371500&r2=371501&view=diff
==============================================================================
--- llvm/trunk/test/tools/yaml2obj/program-header-align.yaml (original)
+++ llvm/trunk/test/tools/yaml2obj/program-header-align.yaml Tue Sep 10 02:16:34 2019
@@ -1,63 +1,56 @@
-# RUN: yaml2obj %s -o %t
+## Test the p_align field of a program header can be set explicitly or inferred
+## from the maximum alignment of contained sections.
+
+## Explicit Align has priority over section alignments.
+
+# RUN: yaml2obj --docnum=1 %s -o %t
 # RUN: llvm-readobj -l %t | FileCheck %s
+# CHECK:      ProgramHeader {
+# CHECK-NOT:  ProgramHeader {
+# CHECK:        Alignment: 16
+# CHECK-NEXT: }
 
-!ELF
+--- !ELF
 FileHeader:
-  Class:           ELFCLASS64
-  Data:            ELFDATA2LSB
-  Type:            ET_EXEC
-  Machine:         EM_X86_64
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_NONE
 Sections:
-  - Name:            .text
-    Type:            SHT_PROGBITS
-    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
-    AddressAlign:    0x0000000000000008
-    Content:         "00000000"
-  - Name:            .data
-    Type:            SHT_PROGBITS
-    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
-    Content:         "00000000"
-    AddressAlign:    0x0000000000000010
+  - Name:         .tdata
+    Type:         SHT_PROGBITS
+    AddressAlign: 4
+  - Name:         .tbss
+    Type:         SHT_NOBITS
+    AddressAlign: 64
 ProgramHeaders:
-  - Type: PT_LOAD
-    Flags: [ PF_X, PF_R ]
-    VAddr: 0xAAAA1030
-    PAddr: 0xFFFF1030
-    Align: 0x1000
+  - Type:  PT_TLS
+    Align: 16
     Sections:
-      - Section: .text
+      - Section: .tdata
+      - Section: .tbss
+
+## If Align is not specified, p_align is inferred from the maximum alignment
+## of contained sections.
+
+# RUN: yaml2obj --docnum=2 %s -o %t
+# RUN: llvm-readobj -l %t | FileCheck %s
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_EXEC
+  Machine: EM_NONE
+Sections:
+  - Name:         .text
+    Type:         SHT_PROGBITS
+    AddressAlign: 4
+  - Name:         .text.hot
+    Type:         SHT_PROGBITS
+    AddressAlign: 16
+ProgramHeaders:
   - Type: PT_LOAD
-    Flags: [ PF_R ]
-    VAddr: 0xAAAA2040
-    PAddr: 0xFFFF2040
-    Align: 0x1000
     Sections:
-      - Section: .data
-
-#CHECK:     ProgramHeaders [
-#CHECK-NEXT:   ProgramHeader {
-#CHECK-NEXT:    Type: PT_LOAD
-#CHECK-NEXT:    Offset: 0xB0
-#CHECK-NEXT:    VirtualAddress: 0xAAAA1030
-#CHECK-NEXT:    PhysicalAddress: 0xFFFF1030
-#CHECK-NEXT:    FileSize: 4
-#CHECK-NEXT:    MemSize: 4
-#CHECK-NEXT:    Flags [
-#CHECK-NEXT:      PF_R
-#CHECK-NEXT:      PF_X
-#CHECK-NEXT:    ]
-#CHECK-NEXT:    Alignment: 4096
-#CHECK-NEXT:  }
-#CHECK-NEXT:  ProgramHeader {
-#CHECK-NEXT:    Type: PT_LOAD
-#CHECK-NEXT:    Offset: 0xC0
-#CHECK-NEXT:    VirtualAddress: 0xAAAA2040
-#CHECK-NEXT:    PhysicalAddress: 0xFFFF2040
-#CHECK-NEXT:    FileSize: 4
-#CHECK-NEXT:    MemSize: 4
-#CHECK-NEXT:    Flags [
-#CHECK-NEXT:      PF_R
-#CHECK-NEXT:    ]
-#CHECK-NEXT:    Alignment: 4096
-#CHECK-NEXT:  }
-#CHECK-NEXT:]
+      - Section: .text
+      - Section: .text.hot