[ELF] - Add testcase documenting flags assigned when empty synthetic section is removed.

[grimar]

Summary

This responds to PR36475,

r325763 led to unexprected layout change, though
new behavior seems to be more correct.

Previously we could have following script:

.foo : { *(.foo) } 
.bar : { *(.synthetic_empty) BYTE(0x11) }}

where synthetic_empty is a synthetic section which is empty and
hence removed by linker.
Before r325763 .bar would receive section flags from .synthetic_empty,
but after this revision it receives flags the same as .foo section has.

It is the same as if there would not be any synthetic_empty section in a script,
so looks reasonable and consistent behavior:

.foo : { *(.foo) } 
.bar : { BYTE(0x11) }}

Patch adds testcase to document it.

[ruiu]

LGTM with this fix.

test/ELF/linkerscript/empty-synthetic-removed-flags.s

3–5

For multi-line linker script, can you avoid using \ for line continuation? I found that that is hard to debug when something goes wrong, because this "echo" ends up with a very long single line with a lot of whitespace characters, and "^" marker doesn't make sense in such output because lines are wrapped around.

You could instead write

echo "SECTIONS { .foo : { *(.foo) }" > %t.script
echo "           .bar : { *(.got.plt) BYTE(0x11) }}" >> %t.script

That said, since this example is short, the best way of doing it is to write it in one line.

echo "SECTIONS { .foo : { *(.foo) } .bar : { *(.got.plt) BYTE(0x11) }}" > %t.script

25–27

Ditto

test/ELF/linkerscript/empty-synthetic-removed-flags.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS {                          \
+# RUN:         .foo : { *(.foo) }                \
+# RUN:         .bar : { *(.got.plt) BYTE(0x11) }}" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+# RUN: llvm-readobj -s %t | FileCheck %s
+
+## We have ".got.plt" synthetic section with SHF_ALLOC|SHF_WRITE flags.
+## It is empty, so linker removes it, but it have to keep ".got.plt" output
+## section because of BYTE command. Here we check that result output section
+## gets the same flags as previous allocatable section and does not get
+## SHF_WRITE flag from removed syntethic input section.
+
+# CHECK:     Section {
+# CHECK:       Index: 2
+# CHECK:       Name: .bar
+# CHECK-NEXT:  Type: SHT_PROGBITS
+# CHECK-NEXT:  Flags [
+# CHECK-NEXT:    SHF_ALLOC
+# CHECK-NEXT:    SHF_EXECINSTR
+# CHECK-NEXT:  ]
+
+## Check flags are the same if we omit empty synthetic section in script.
+# RUN: echo "SECTIONS {              \
+# RUN:         .foo : { *(.foo) }    \
+# RUN:         .bar : { BYTE(0x11) }}" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+# RUN: llvm-readobj -s %t | FileCheck %s
+
+.section .foo,"ax"
+.quad 0