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[BOLT] Fix debug info for input with continuous range. 

Summary:
When we see a compilation unit with continuous range on input,
it has two attributes: DW_AT_low_pc and DW_AT_high_pc. We convert the
range to a non-continuous one and change the attributes to
DW_AT_ranges and DW_AT_producer. However, gdb seems to expect
every compilation unit to have a base address specified via
DW_AT_low_pc, even when its value is always 0. Otherwise gdb will
not show proper debug info for such modules.

With this diff we produce DW_AT_ranges followed by DW_AT_low_pc.
The problem is that the first attribute takes DW_FORM_sec_offset
which is exactly 4 bytes, and in many cases we are left with
12 bytes to fill in. We used to fill this space with DW_AT_producer,
which took an arbitrary-length field. For DW_AT_low_pc we can
use a trick of using DW_FORM_udata (unsigned ULEB128 encoded
integer) which can take up to 12 bytes, even when the value is 0.

(cherry picked from FBD5109798)
This commit is contained in:
Maksim Panchenko 2017-05-22 17:17:04 -07:00
parent 4806b13835
commit 457b7f14b9
3 changed files with 41 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

51
bolt/DWARFRewriter.cpp
View File

@ -174,15 +174,18 @@ void RewriteInstance::updateDWARFObjectAddressRanges(
DIE->getAttributeValue(Unit, dwarf::DW_AT_ranges, FormValue, &AttrOffset);
DebugInfoPatcher->addLE32Patch(AttrOffset, DebugRangesOffset);
} else {
// Case 2: The object has both DW_AT_low_pc and DW_AT_high_pc.
// We require the compiler to put both attributes one after the other
// for our approach to work. low_pc and high_pc both occupy 8 bytes
// as we're dealing with a 64-bit ELF. We basically change low_pc to
// DW_AT_ranges and high_pc to DW_AT_producer. ranges spans only 4 bytes
// in 32-bit DWARF, which we assume to be used, which leaves us with 12
// more bytes. We then set the value of DW_AT_producer as an arbitrary
// 12-byte string that fills the remaining space and leaves the rest of
// the abbreviation layout unchanged.
// Case 2: The object has both DW_AT_low_pc and DW_AT_high_pc emitted back
// to back. We replace the attributes with DW_AT_ranges and DW_AT_low_pc.
// The low_pc attribute is required for DW_TAG_compile_units to set a base
// address.
//
// Since DW_AT_ranges takes 4-byte DW_FROM_sec_offset value, we have to fill
// in up to 12-bytes left after removal of low/high pc field from
// .debug_info.
//
// To fill in the gap we use a variable length DW_FORM_udata encoding for
// DW_AT_low_pc. We exploit the fact that the encoding can take an arbitrary
// large size.
if (AbbreviationDecl->findAttributeIndex(dwarf::DW_AT_low_pc) != -1U &&
AbbreviationDecl->findAttributeIndex(dwarf::DW_AT_high_pc) != -1U) {
uint32_t LowPCOffset = -1U;
@ -197,19 +200,15 @@ void RewriteInstance::updateDWARFObjectAddressRanges(
(HighPCFormValue.getForm() != dwarf::DW_FORM_addr &&
HighPCFormValue.getForm() != dwarf::DW_FORM_data8 &&
HighPCFormValue.getForm() != dwarf::DW_FORM_data4)) {
if (opts::Verbosity >= 1) {
errs() << "BOLT-WARNING: unexpected form value. Cannot update DIE "
errs() << "BOLT-WARNING: unexpected form value. Cannot update DIE "
<< "at offset 0x" << Twine::utohexstr(DIE->getOffset())
<< "\n";
}
return;
}
if (LowPCOffset == -1U || (LowPCOffset + 8 != HighPCOffset)) {
if (opts::Verbosity >= 1) {
errs() << "BOLT-WARNING: high_pc expected immediately after low_pc. "
<< "Cannot update DIE at offset 0x"
<< Twine::utohexstr(DIE->getOffset()) << '\n';
}
errs() << "BOLT-WARNING: high_pc expected immediately after low_pc. "
<< "Cannot update DIE at offset 0x"
<< Twine::utohexstr(DIE->getOffset()) << '\n';
return;
}
@ -221,23 +220,19 @@ void RewriteInstance::updateDWARFObjectAddressRanges(
AbbrevPatcher->addAttributePatch(Unit,
AbbrevCode,
dwarf::DW_AT_high_pc,
dwarf::DW_AT_producer,
dwarf::DW_FORM_string);
unsigned StringSize = 0;
dwarf::DW_AT_low_pc,
dwarf::DW_FORM_udata);
unsigned LowPCSize = 0;
if (HighPCFormValue.getForm() == dwarf::DW_FORM_addr ||
HighPCFormValue.getForm() == dwarf::DW_FORM_data8) {
StringSize = 12;
LowPCSize = 12;
} else if (HighPCFormValue.getForm() == dwarf::DW_FORM_data4) {
StringSize = 8;
LowPCSize = 8;
} else {
assert(0 && "unexpected form");
llvm_unreachable("unexpected form");
}
DebugInfoPatcher->addLE32Patch(LowPCOffset, DebugRangesOffset);
std::string ProducerString{"LLVM-BOLT"};
ProducerString.resize(StringSize, ' ');
ProducerString.back() = '\0';
DebugInfoPatcher->addBinaryPatch(LowPCOffset + 4, ProducerString);
DebugInfoPatcher->addUDataPatch(LowPCOffset + 4, 0, LowPCSize);
} else {
if (opts::Verbosity >= 1) {
errs() << "BOLT-WARNING: Cannot update ranges for DIE at offset 0x"

13
bolt/DebugData.cpp
View File

@ -15,6 +15,7 @@
#include "llvm/MC/MCSymbol.h"
#include "llvm/MC/MCObjectWriter.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/LEB128.h"
#include <algorithm>
#include <cassert>
@ -270,6 +271,18 @@ void SimpleBinaryPatcher::addLEPatch(uint32_t Offset, uint64_t NewValue,
Patches.emplace_back(std::make_pair(Offset, LE64));
}
void SimpleBinaryPatcher::addUDataPatch(uint32_t Offset, uint64_t Value, uint64_t Size) {
const auto EncodedSize = getULEB128Size(Value);
assert(EncodedSize <= Size && "value did not fit");
const auto Padding = Size - EncodedSize;
std::string Buff;
raw_string_ostream OS(Buff);
encodeULEB128(Value, OS, Padding);
Patches.emplace_back(Offset, OS.str());
}
void SimpleBinaryPatcher::addLE64Patch(uint32_t Offset, uint64_t NewValue) {
addLEPatch(Offset, NewValue, 8);
}

5
bolt/DebugData.h
View File

@ -345,6 +345,11 @@ public:
/// little-endian value at offset \p Offset.
void addLE32Patch(uint32_t Offset, uint32_t NewValue);
/// Add a patch at \p Offset with \p Value using unsigned LEB128 encoding with
/// size \p Size. \p Size should not be less than a minimum number of bytes
/// needed to encode \p Value.
void addUDataPatch(uint32_t Offset, uint64_t Value, uint64_t Size);
void patchBinary(std::string &BinaryContents) override;
};