Merge .xdata into .rdata by default

[rnk]

Summary

MSVC does this. The user can override it with their own /merge: flag.

[rnk]

Do you think we should merge .idata, .edata, and .didata into .rdata as well? That will be more involved than this.

[smeenai]

What are the advantages to the section merging, apart from matching link's ouput? I personally prefer LLD keeping the separate sections, since it's easier to examine them that way.

[rnk]

In D40197#929200, @smeenai wrote:

What are the advantages to the section merging, apart from matching link's ouput? I personally prefer LLD keeping the separate sections, since it's easier to examine them that way.

Saves alignment padding, mostly. All of .xdata, .edata, .idata, and .didata can be dumped by llvm-readobj by starting from the PE headers, so they aren't that hard to read without section headers. The alignment padding is particularly bad when sections have different protections so we have to pad out to 4096 to get to a page boundary, as in .data vs. .didata, which are typically near each other because we sort output sections alphabetically.

[smeenai]

Ah all right, that makes sense then.

[ruiu]

LGTM

[rnk]

This caused EH-related test failures on 32-bit LLD bots: https://ci.chromium.org/buildbot/chromium.clang/CrWinClangLLD/166

64-bit code was fine. I'm reverting and will investigate later.

lld/trunk/COFF/Driver.cpp

void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   // Handle /failifmismatch
   for (auto *Arg : Args.filtered(OPT_failifmismatch))
     checkFailIfMismatch(Arg->getValue());
 
   // Handle /merge
   for (auto *Arg : Args.filtered(OPT_merge))
     parseMerge(Arg->getValue());
 
+  // Add default section merging rules after user rules. User rules take
+  // precedence, but we will emit a warning if there is a conflict.
+  parseMerge(".xdata=.rdata");
+
   // Handle /section
   for (auto *Arg : Args.filtered(OPT_section))
     parseSection(Arg->getValue());
 
   // Handle /aligncomm
   for (auto *Arg : Args.filtered(OPT_aligncomm))
     parseAligncomm(Arg->getValue());
 

lld/trunk/test/COFF/icf-xdata.s

 # RUN: llvm-mc %s -triple x86_64-windows-msvc -filetype=obj -o %t.obj
+# RUN: lld-link %t.obj -dll -noentry -out:%t.dll -merge:.xdata=.xdata 2>&1 \
+# RUN:     | FileCheck %s --check-prefix=WARN
+# RUN: llvm-readobj -sections %t.dll | FileCheck %s --check-prefix=XDATA
 # RUN: lld-link %t.obj -dll -noentry -out:%t.dll
-# RUN: llvm-readobj -sections %t.dll | FileCheck %s
+# RUN: llvm-readobj -sections %t.dll | FileCheck %s --check-prefix=RDATA
 
 # There shouldn't be much xdata, because all three .pdata entries (12 bytes
 # each) should use the same .xdata unwind info.
-# CHECK:         Name: .pdata
-# CHECK-NEXT:    VirtualSize: 0x24
-# CHECK:         Name: .xdata
-# CHECK-NEXT:    VirtualSize: 0x8
+# XDATA:         Name: .pdata
+# XDATA-NEXT:    VirtualSize: 0x24
+# XDATA:         Name: .xdata
+# XDATA-NEXT:    VirtualSize: 0x8
+#
+# WARN: warning: .xdata=.rdata: already merged into .xdata
+#
+# RDATA:         Name: .pdata
+# RDATA-NEXT:    VirtualSize: 0x24
+# RDATA:         Name: .rdata
+# RDATA-NEXT:    VirtualSize: 0x8
 
   .text
 callee:
   ret
 
   .def   xdata1;
   .scl  2;
   .type 32;

lld/trunk/test/COFF/pdb.test

 RAW-NEXT:     off = 0
 RAW:                            Section Headers
 RAW-NEXT: ============================================================
 RAW:        SECTION HEADER #1
 RAW-NEXT:     .pdata name
 RAW-NEXT:            virtual size
 RAW-NEXT:       1000 virtual address
 RAW-NEXT:        200 size of raw data
-RAW-NEXT:        400 file pointer to raw data
+RAW-NEXT:        200 file pointer to raw data
 RAW-NEXT:          0 file pointer to relocation table
 RAW-NEXT:          0 file pointer to line numbers
 RAW-NEXT:          0 number of relocations
 RAW-NEXT:          0 number of line numbers
 RAW-NEXT:   40000040 flags
 RAW-NEXT:            IMAGE_SCN_CNT_INITIALIZED_DATA
 RAW-NEXT:            IMAGE_SCN_MEM_READ
 RAW:        SECTION HEADER #2
 RAW-NEXT:      .text name
 RAW-NEXT:            virtual size
 RAW-NEXT:       2000 virtual address
 RAW-NEXT:        200 size of raw data
-RAW-NEXT:        600 file pointer to raw data
+RAW-NEXT:        400 file pointer to raw data
 RAW-NEXT:          0 file pointer to relocation table
 RAW-NEXT:          0 file pointer to line numbers
 RAW-NEXT:          0 number of relocations
 RAW-NEXT:          0 number of line numbers
 RAW-NEXT:   60000020 flags
 RAW-NEXT:            IMAGE_SCN_CNT_CODE
 RAW-NEXT:            IMAGE_SCN_MEM_EXECUTE
 RAW-NEXT:            IMAGE_SCN_MEM_READ
 RAW:        SECTION HEADER #3
-RAW-NEXT:     .xdata name
-RAW-NEXT:            virtual size
-RAW-NEXT:       3000 virtual address
-RAW-NEXT:        200 size of raw data
-RAW-NEXT:        800 file pointer to raw data
-RAW-NEXT:          0 file pointer to relocation table
-RAW-NEXT:          0 file pointer to line numbers
-RAW-NEXT:          0 number of relocations
-RAW-NEXT:          0 number of line numbers
-RAW-NEXT:   40000040 flags
-RAW-NEXT:            IMAGE_SCN_CNT_INITIALIZED_DATA
-RAW-NEXT:            IMAGE_SCN_MEM_READ
-RAW:        SECTION HEADER #4
 RAW-NEXT:     .rdata name
 RAW-NEXT:            virtual size
-RAW-NEXT:       4000 virtual address
+RAW-NEXT:       3000 virtual address
 RAW-NEXT:        200 size of raw data
-RAW-NEXT:        A00 file pointer to raw data
+RAW-NEXT:        600 file pointer to raw data
 RAW-NEXT:          0 file pointer to relocation table
 RAW-NEXT:          0 file pointer to line numbers
 RAW-NEXT:          0 number of relocations
 RAW-NEXT:          0 number of line numbers
 RAW-NEXT:   40000040 flags
 RAW-NEXT:            IMAGE_SCN_CNT_INITIALIZED_DATA
 RAW-NEXT:            IMAGE_SCN_MEM_READ
+RAW-NOT:        SECTION HEADER
 RAW:                        Original Section Headers
 RAW-NEXT: ============================================================
 RAW-NEXT:   PDB does not contain the requested image section header type
 RAW:                        Section Contributions
 RAW-NEXT: ============================================================
 RAW-NEXT:   SC[.pdata]  | mod = 0, 0001:0000, size = 12, data crc = 361370162, reloc crc = 0
 RAW-NEXT:                 IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_ALIGN_4BYTES | IMAGE_SCN_MEM_READ
 RAW-NEXT:   SC[.text]   | mod = 0, 0002:0000, size = 14, data crc = 1682752513, reloc crc = 0
 RAW-NEXT:                 IMAGE_SCN_CNT_CODE | IMAGE_SCN_ALIGN_16BYTES | IMAGE_SCN_MEM_EXECUTE |
 RAW-NEXT:                 IMAGE_SCN_MEM_READ
 RAW-NEXT:   SC[.text]   | mod = 1, 0002:0016, size = 6, data crc = 2139436471, reloc crc = 0
 RAW-NEXT:                 IMAGE_SCN_CNT_CODE | IMAGE_SCN_ALIGN_16BYTES | IMAGE_SCN_MEM_EXECUTE |
 RAW-NEXT:                 IMAGE_SCN_MEM_READ
-RAW-NEXT:   SC[.xdata]  | mod = 0, 0003:0000, size = 8, data crc = 264583633, reloc crc = 0
+RAW-NEXT:   SC[???]     | mod = 0, 0003:0000, size = 8, data crc = 264583633, reloc crc = 0
 RAW-NEXT:                 IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_ALIGN_4BYTES | IMAGE_SCN_MEM_READ
+RAW-NEXT:   SC[???]     | mod = 2, 0003:0008, size = 28, data crc = 0, reloc crc = 0
+RAW-NEXT:                 IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ
+RAW-NEXT:   SC[???]     | mod = 2, 0003:0036, size = 94, data crc = 0, reloc crc = 0
+RAW-NEXT:                 IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ
+RAW-NOT: SC[
 RAW:                             Section Map
 RAW-NEXT: ============================================================
 RAW-NEXT:   Section 0000 | ovl = 0, group = 0, frame = 1, name = 65535
 RAW-NEXT:                  class = 65535, offset = 0, size =
 RAW-NEXT:                  flags = read | 32 bit addr | selector
 RAW-NEXT:   Section 0001 | ovl = 0, group = 0, frame = 2, name = 65535
 RAW-NEXT:                  class = 65535, offset = 0, size =
 RAW-NEXT:                  flags = read | execute | 32 bit addr | selector
 RAW-NEXT:   Section 0002 | ovl = 0, group = 0, frame = 3, name = 65535
 RAW-NEXT:                  class = 65535, offset = 0, size =
 RAW-NEXT:                  flags = read | 32 bit addr | selector
 RAW-NEXT:   Section 0003 | ovl = 0, group = 0, frame = 4, name = 65535
 RAW-NEXT:                  class = 65535, offset = 0, size =
-RAW-NEXT:                  flags = read | 32 bit addr | selector
-RAW-NEXT:   Section 0004 | ovl = 0, group = 0, frame = 5, name = 65535
-RAW-NEXT:                  class = 65535, offset = 0, size =
 RAW-NEXT:                  flags = 32 bit addr | absolute addr