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llvm-project/lld/test/COFF/autoimport-list-ptrs.s

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[COFF] Support MinGW automatic dllimport of data Normally, in order to reference exported data symbols from a different DLL, the declarations need to have the dllimport attribute, in order to use the __imp_<var> symbol (which contains an address to the actual variable) instead of the variable itself directly. This isn't an issue in the same way for functions, since any reference to the function without the dllimport attribute will end up as a reference to a thunk which loads the actual target function from the import address table (IAT). GNU ld, in MinGW environments, supports automatically importing data symbols from DLLs, even if the references didn't have the appropriate dllimport attribute. Since the PE/COFF format doesn't support the kind of relocations that this would require, the MinGW's CRT startup code has an custom framework of their own for manually fixing the missing relocations once module is loaded and the target addresses in the IAT are known. For this to work, the linker (originall in GNU ld) creates a list of remaining references needing fixup, which the runtime processes on startup before handing over control to user code. While this feature is rather controversial, it's one of the main features allowing unix style libraries to be used on windows without any extra porting effort. Some sort of automatic fixing of data imports is also necessary for the itanium C++ ABI on windows (as clang implements it right now) for importing vtable pointers in certain cases, see D43184 for some discussion on that. The runtime pseudo relocation handler supports 8/16/32/64 bit addresses, either PC relative references (like IMAGE_REL_*_REL32*) or absolute references (IMAGE_REL_AMD64_ADDR32, IMAGE_REL_AMD64_ADDR32, IMAGE_REL_I386_DIR32). On linking, the relocation is handled as a relocation against the corresponding IAT slot. For the absolute references, a normal base relocation is created, to update the embedded address in case the image is loaded at a different address. The list of runtime pseudo relocations contains the RVA of the imported symbol (the IAT slot), the RVA of the location the relocation should be applied to, and a size of the memory location. When the relocations are fixed at runtime, the difference between the actual IAT slot value and the IAT slot address is added to the reference, doing the right thing for both absolute and relative references. With this patch alone, things work fine for i386 binaries, and mostly for x86_64 binaries, with feature parity with GNU ld. Despite this, there are a few gotchas: - References to data from within code works fine on both x86 architectures, since their relocations consist of plain 32 or 64 bit absolute/relative references. On ARM and AArch64, references to data doesn't consist of a plain 32 or 64 bit embedded address or offset in the code. On ARMNT, it's usually a MOVW+MOVT instruction pair represented by a IMAGE_REL_ARM_MOV32T relocation, each instruction containing 16 bit of the target address), on AArch64, it's usually an ADRP+ADD/LDR/STR instruction pair with an even more complex encoding, storing a PC relative address (with a range of +/- 4 GB). This could theoretically be remedied by extending the runtime pseudo relocation handler with new relocation types, to support these instruction encodings. This isn't an issue for GCC/GNU ld since they don't support windows on ARMNT/AArch64. - For x86_64, if references in code are encoded as 32 bit PC relative offsets, the runtime relocation will fail if the target turns out to be out of range for a 32 bit offset. - Fixing up the relocations at runtime requires making sections writable if necessary, with the VirtualProtect function. In Windows Store/UWP apps, this function is forbidden. These limitations are addressed by a few later patches in lld and llvm. Differential Revision: https://reviews.llvm.org/D50917 llvm-svn: 340726
2018-08-27 16:43:31 +08:00
# REQUIRES: x86
# RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.obj
# RUN: lld-link -lldmingw -out:%t.exe -entry:main %t.obj -verbose
# RUN: llvm-objdump -s %t.exe | FileCheck -check-prefix=CONTENTS %s
# Even if we didn't actually write any pseudo relocations,
# check that the synthetic pointers still are set to a non-null value
# CONTENTS: Contents of section .data:
[COFF] Provide __CTOR_LIST__ and __DTOR_LIST__ symbols for MinGW MinGW uses these kind of list terminator symbols for traversing the constructor/destructor lists. These list terminators are actual pointers entries in the lists, with the values 0 and (uintptr_t)-1 (instead of just symbols pointing to the start/end of the list). (This mechanism exists in both the mingw-w64 crt startup code and in libgcc; normally the mingw-w64 one is used, but a DLL build of libgcc uses the libgcc one. Therefore it's not trivial to change the mechanism without lots of cross-project synchronization and potentially invalidating some combinations of old/new versions of them.) When mingw-w64 has been used with lld so far, the CRT startup object files have so far provided these symbols, ending up with different, incompatible builds of the CRT startup object files depending on whether binutils or lld are going to be used. In order to avoid the need of different configuration of the CRT startup object files depending on what linker to be used, provide these symbols in lld instead. (Mingw-w64 checks at build time whether the linker provides these symbols or not.) This unifies this particular detail between the two linkers. This does disallow the use of the very latest lld with older versions of mingw-w64 (the configure check for the list was added recently; earlier it simply checked whether the CRT was built with gcc or clang), and requires rebuilding the mingw-w64 CRT. But the number of users of lld+mingw still is low enough that such a change should be tolerable, and unifies this aspect of the toolchains, easing interoperability between the toolchains for the future. The actual test for this feature is added in ctors_dtors_priority.s, but a number of other tests that checked absolute output addresses are updated. Differential Revision: https://reviews.llvm.org/D52053 llvm-svn: 342294
2018-09-15 06:26:59 +08:00
# CONTENTS: 140003000 00200040 01000000 00200040 01000000
[COFF] Support MinGW automatic dllimport of data Normally, in order to reference exported data symbols from a different DLL, the declarations need to have the dllimport attribute, in order to use the __imp_<var> symbol (which contains an address to the actual variable) instead of the variable itself directly. This isn't an issue in the same way for functions, since any reference to the function without the dllimport attribute will end up as a reference to a thunk which loads the actual target function from the import address table (IAT). GNU ld, in MinGW environments, supports automatically importing data symbols from DLLs, even if the references didn't have the appropriate dllimport attribute. Since the PE/COFF format doesn't support the kind of relocations that this would require, the MinGW's CRT startup code has an custom framework of their own for manually fixing the missing relocations once module is loaded and the target addresses in the IAT are known. For this to work, the linker (originall in GNU ld) creates a list of remaining references needing fixup, which the runtime processes on startup before handing over control to user code. While this feature is rather controversial, it's one of the main features allowing unix style libraries to be used on windows without any extra porting effort. Some sort of automatic fixing of data imports is also necessary for the itanium C++ ABI on windows (as clang implements it right now) for importing vtable pointers in certain cases, see D43184 for some discussion on that. The runtime pseudo relocation handler supports 8/16/32/64 bit addresses, either PC relative references (like IMAGE_REL_*_REL32*) or absolute references (IMAGE_REL_AMD64_ADDR32, IMAGE_REL_AMD64_ADDR32, IMAGE_REL_I386_DIR32). On linking, the relocation is handled as a relocation against the corresponding IAT slot. For the absolute references, a normal base relocation is created, to update the embedded address in case the image is loaded at a different address. The list of runtime pseudo relocations contains the RVA of the imported symbol (the IAT slot), the RVA of the location the relocation should be applied to, and a size of the memory location. When the relocations are fixed at runtime, the difference between the actual IAT slot value and the IAT slot address is added to the reference, doing the right thing for both absolute and relative references. With this patch alone, things work fine for i386 binaries, and mostly for x86_64 binaries, with feature parity with GNU ld. Despite this, there are a few gotchas: - References to data from within code works fine on both x86 architectures, since their relocations consist of plain 32 or 64 bit absolute/relative references. On ARM and AArch64, references to data doesn't consist of a plain 32 or 64 bit embedded address or offset in the code. On ARMNT, it's usually a MOVW+MOVT instruction pair represented by a IMAGE_REL_ARM_MOV32T relocation, each instruction containing 16 bit of the target address), on AArch64, it's usually an ADRP+ADD/LDR/STR instruction pair with an even more complex encoding, storing a PC relative address (with a range of +/- 4 GB). This could theoretically be remedied by extending the runtime pseudo relocation handler with new relocation types, to support these instruction encodings. This isn't an issue for GCC/GNU ld since they don't support windows on ARMNT/AArch64. - For x86_64, if references in code are encoded as 32 bit PC relative offsets, the runtime relocation will fail if the target turns out to be out of range for a 32 bit offset. - Fixing up the relocations at runtime requires making sections writable if necessary, with the VirtualProtect function. In Windows Store/UWP apps, this function is forbidden. These limitations are addressed by a few later patches in lld and llvm. Differential Revision: https://reviews.llvm.org/D50917 llvm-svn: 340726
2018-08-27 16:43:31 +08:00
.global main
.text
main:
retq
.data
relocs:
.quad __RUNTIME_PSEUDO_RELOC_LIST__
.quad __RUNTIME_PSEUDO_RELOC_LIST_END__