[ELF] Allow sh_entsize to be unrelated to sh_addralign and not a power of 2

[MaskRay]

Summary

This implements Rui Ueyama's idea in https://bugs.llvm.org/show_bug.cgi?id=39044#c3
I've checked that ld.bfd and gold do not have the power-of-2 requirement
and do not require sh_entsize to be a multiple of sh_align.

Now on the updated test merge-entsize.s, all the 3 linkers happily
create .rodata that is not 3-byte aligned.

This has a use case in Linux arch/x86/crypto/sha512-avx2-asm.S
It uses sh_entsize of 640, which is not a power of 2.
See https://github.com/ClangBuiltLinux/linux/issues/417

[MaskRay]

Update title

[MaskRay]

As a side note, sh_size%sh_entsize==0 is required and that is tested by test/ELF/invalid/merge-invalid-size.s

[ruiu]

LGTM

lld/trunk/ELF/SyntheticSections.cpp

for (InputSectionBase *&S : InputSections) {
     // We do not want to handle sections that are not alive, so just remove
     // them instead of trying to merge.
     if (!MS->Live) {
       S = nullptr;
       continue;
     }
 
     StringRef OutsecName = getOutputSectionName(MS);
-    uint32_t Alignment = std::max<uint32_t>(MS->Alignment, MS->Entsize);
 
     auto I = llvm::find_if(MergeSections, [=](MergeSyntheticSection *Sec) {
       // While we could create a single synthetic section for two different
       // values of Entsize, it is better to take Entsize into consideration.
       //
       // With a single synthetic section no two pieces with different Entsize
       // could be equal, so we may as well have two sections.
       //
       // Using Entsize in here also allows us to propagate it to the synthetic
       // section.
       return Sec->Name == OutsecName && Sec->Flags == MS->Flags &&
-             Sec->Entsize == MS->Entsize && Sec->Alignment == Alignment;
+             Sec->Entsize == MS->Entsize && Sec->Alignment == MS->Alignment;
     });
     if (I == MergeSections.end()) {
       MergeSyntheticSection *Syn =
-          createMergeSynthetic(OutsecName, MS->Type, MS->Flags, Alignment);
+          createMergeSynthetic(OutsecName, MS->Type, MS->Flags, MS->Alignment);
       MergeSections.push_back(Syn);
       I = std::prev(MergeSections.end());
       S = Syn;
       Syn->Entsize = MS->Entsize;
     } else {
       S = nullptr;
     }
     (*I)->addSection(MS);

lld/trunk/test/ELF/icf-merged-sections.s

 # CHECK-NEXT:   SHF_ALLOC
 # CHECK-NEXT:   SHF_MERGE
 # CHECK-NEXT: ]
 # CHECK-NEXT: Address:
 # CHECK-NEXT: Offset:
 # CHECK-NEXT: Size: 16
 # CHECK-NEXT: Link:
 # CHECK-NEXT: Info:
-# CHECK-NEXT: AddressAlignment: 8
+# CHECK-NEXT: AddressAlignment: 1
 # CHECK-NEXT: EntrySize: 0
 # CHECK-NEXT: SectionData (
 # CHECK-NEXT:   0000: 67452301 10325476 67452301 10325476
 
 .section .rodata.cst4,"aM",@progbits,4
 rodata4:
   .long 0x01234567
   .long 0x76543210
   .long 0x01234567
   .long 0x76543210
 
 .section .rodata.cst8,"aM",@progbits,8
 rodata8:
   .long 0x01234567
   .long 0x76543210

lld/trunk/test/ELF/merge-entsize.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 // RUN: ld.lld %t.o -o %t
 // RUN: llvm-readobj -s %t | FileCheck %s
 
         .section  .rodata.1,"aM",@progbits,1
+        .p2align 2
         .byte 0x42
 
-        .section  .rodata.2,"aM",@progbits,2
+// sh_addralign = 4 while sh_entsize = 3.
+// sh_entsize is not necessarily a power of 2 and it can be unrelated to sh_addralign.
+        .section  .rodata.2,"aM",@progbits,3
+        .p2align 2
         .short 0x42
+        .byte 0
 
 // Since the output section has both .rodata.1 and .rodata.2, it
 // contains elements of different sizes and we use an entsize of 0.
 
 // CHECK:      Name: .rodata (
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT:   SHF_MERGE
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address:
 // CHECK-NEXT: Offset:
 // CHECK-NEXT: Size:
 // CHECK-NEXT: Link:
 // CHECK-NEXT: Info:
-// CHECK-NEXT: AddressAlignment:
+// CHECK-NEXT: AddressAlignment: 4
 // CHECK-NEXT: EntrySize: 0

lld/trunk/test/ELF/merge-gc-piece.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: ld.lld %t.o -o %t.so -shared --gc-sections
 # RUN: llvm-readobj -s -section-data %t.so | FileCheck %s
 
 
 # CHECK:      Name: .foo
 # CHECK-NEXT: Type: SHT_PROGBITS
 # CHECK-NEXT: Flags [
 # CHECK-NEXT:   SHF_ALLOC
 # CHECK-NEXT:   SHF_MERGE
 # CHECK-NEXT: ]
-# CHECK-NEXT: Address: 0x200
+# CHECK-NEXT: Address: 0x1FD
 
 # CHECK:      Name: .bar
 # CHECK-NEXT: Type: SHT_PROGBITS
 # CHECK-NEXT: Flags [
 # CHECK-NEXT: ]
 # CHECK-NEXT: Address:
 # CHECK-NEXT: Offset:
 # CHECK-NEXT: Size: 16
 # CHECK-NEXT: Link:
 # CHECK-NEXT: Info:
 # CHECK-NEXT: AddressAlignment:
 # CHECK-NEXT: EntrySize:
 # CHECK-NEXT: SectionData (
-# CHECK-NEXT:   0000: 01020000 00000000 02020000 00000000
+# CHECK-NEXT:   0000: FE010000 00000000 FF010000 00000000
 # CHECK-NEXT: )
 
         .section .foo,"aM",@progbits,8
         .quad 42
         .global sym
 sym:
         .quad 43
 
         .section .bar
         .quad .foo + 1
         .quad .foo + 2

lld/trunk/test/ELF/merge-reloc-O0.s

 # CHECK-NEXT:   SHF_ALLOC
 # CHECK-NEXT:   SHF_MERGE
 # CHECK-NEXT: ]
 # CHECK-NEXT: Address:
 # CHECK-NEXT: Offset:
 # CHECK-NEXT: Size: 16
 # CHECK-NEXT: Link:
 # CHECK-NEXT: Info:
-# CHECK-NEXT: AddressAlignment: 8
+# CHECK-NEXT: AddressAlignment: 1
 # CHECK-NEXT: EntrySize: 8
 # CHECK-NEXT: SectionData (
 # CHECK-NEXT:   0000: 41000000 00000000 42000000 00000000
 # CHECK-NEXT: )
 
 # CHECK:      Name: .foo
 # CHECK-NEXT: Type: SHT_PROGBITS
 # CHECK-NEXT: Flags [
 # CHECK-NEXT:   SHF_ALLOC
 # CHECK-NEXT:   SHF_MERGE
 # CHECK-NEXT: ]
 # CHECK-NEXT: Address:
 # CHECK-NEXT: Offset:
 # CHECK-NEXT: Size: 8
 # CHECK-NEXT: Link:
 # CHECK-NEXT: Info:
-# CHECK-NEXT: AddressAlignment: 4
+# CHECK-NEXT: AddressAlignment: 1
 # CHECK-NEXT: EntrySize: 4
 # CHECK-NEXT: SectionData (
 # CHECK-NEXT:   0000: 41000000 42000000
 # CHECK-NEXT: )
 
         .section .foo, "aM",@progbits,8,unique,0
         .quad 0x41
         .section .foo, "aM",@progbits,8,unique,1
         .quad 0x42
         .section .foo, "aM",@progbits,4,unique,2
         .long 0x41
         .long 0x42

lld/trunk/test/ELF/merge-sym.s

 
 
 // CHECK:      Name: .rodata
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT:   SHF_MERGE
 // CHECK-NEXT: ]
-// CHECK-NEXT: Address: 0x210
+// CHECK-NEXT: Address: 0x20D
 
 // CHECK:      Name: foo
-// CHECK-NEXT: Value: 0x212
+// CHECK-NEXT: Value: 0x20F