forked from OSchip/llvm-project
06239e42c6
Summary:
The MS ABI convention is that the 'this' pointer on entry is the address
of the vfptr that was used to make the virtual method call. In other
words, the pointer on entry always points to the base subobject that
introduced the virtual method. Consider this hierarchy:
struct A { virtual void f() = 0; };
struct B { virtual void g() = 0; };
struct C : A, B {
void f() override;
void g() override;
};
On entry to C::g, [ER]CX will contain the address of C's B subobject,
and C::g will have to subtract sizeof(A) to recover a pointer to C.
Before this change, we applied this adjustment in the prologue and
stored the new value into the "this" local variable alloca used for
debug info. However, MSVC does not do this, presumably because it is
often profitable to fold the adjustment into later field accesses. This
creates a problem, because the debugger expects the variable to be
unadjusted. Unfortunately, CodeView doesn't have anything like DWARF
expressions for computing variables that aren't in the program anymore,
so we have to declare 'this' to be the unadjusted value if we want the
debugger to see the right value.
This has the side benefit that, in optimized builds, the 'this' pointer
will usually be available on function entry because it doesn't require
any adjustment.
Reviewers: hans
Subscribers: aprantl, cfe-commits
Differential Revision: https://reviews.llvm.org/D40109
llvm-svn: 318440
|
||
|---|---|---|
| .. | ||
| ABIInfo.h | ||
| Address.h | ||
| BackendUtil.cpp | ||
| CGAtomic.cpp | ||
| CGBlocks.cpp | ||
| CGBlocks.h | ||
| CGBuilder.h | ||
| CGBuiltin.cpp | ||
| CGCUDANV.cpp | ||
| CGCUDARuntime.cpp | ||
| CGCUDARuntime.h | ||
| CGCXX.cpp | ||
| CGCXXABI.cpp | ||
| CGCXXABI.h | ||
| CGCall.cpp | ||
| CGCall.h | ||
| CGClass.cpp | ||
| CGCleanup.cpp | ||
| CGCleanup.h | ||
| CGCoroutine.cpp | ||
| CGDebugInfo.cpp | ||
| CGDebugInfo.h | ||
| CGDecl.cpp | ||
| CGDeclCXX.cpp | ||
| CGException.cpp | ||
| CGExpr.cpp | ||
| CGExprAgg.cpp | ||
| CGExprCXX.cpp | ||
| CGExprComplex.cpp | ||
| CGExprConstant.cpp | ||
| CGExprScalar.cpp | ||
| CGGPUBuiltin.cpp | ||
| CGLoopInfo.cpp | ||
| CGLoopInfo.h | ||
| CGObjC.cpp | ||
| CGObjCGNU.cpp | ||
| CGObjCMac.cpp | ||
| CGObjCRuntime.cpp | ||
| CGObjCRuntime.h | ||
| CGOpenCLRuntime.cpp | ||
| CGOpenCLRuntime.h | ||
| CGOpenMPRuntime.cpp | ||
| CGOpenMPRuntime.h | ||
| CGOpenMPRuntimeNVPTX.cpp | ||
| CGOpenMPRuntimeNVPTX.h | ||
| CGRecordLayout.h | ||
| CGRecordLayoutBuilder.cpp | ||
| CGStmt.cpp | ||
| CGStmtOpenMP.cpp | ||
| CGVTT.cpp | ||
| CGVTables.cpp | ||
| CGVTables.h | ||
| CGValue.h | ||
| CMakeLists.txt | ||
| CodeGenABITypes.cpp | ||
| CodeGenAction.cpp | ||
| CodeGenFunction.cpp | ||
| CodeGenFunction.h | ||
| CodeGenModule.cpp | ||
| CodeGenModule.h | ||
| CodeGenPGO.cpp | ||
| CodeGenPGO.h | ||
| CodeGenTBAA.cpp | ||
| CodeGenTBAA.h | ||
| CodeGenTypeCache.h | ||
| CodeGenTypes.cpp | ||
| CodeGenTypes.h | ||
| ConstantEmitter.h | ||
| ConstantInitBuilder.cpp | ||
| CoverageMappingGen.cpp | ||
| CoverageMappingGen.h | ||
| EHScopeStack.h | ||
| ItaniumCXXABI.cpp | ||
| MacroPPCallbacks.cpp | ||
| MacroPPCallbacks.h | ||
| MicrosoftCXXABI.cpp | ||
| ModuleBuilder.cpp | ||
| ObjectFilePCHContainerOperations.cpp | ||
| README.txt | ||
| SanitizerMetadata.cpp | ||
| SanitizerMetadata.h | ||
| SwiftCallingConv.cpp | ||
| TargetInfo.cpp | ||
| TargetInfo.h | ||
| VarBypassDetector.cpp | ||
| VarBypassDetector.h | ||
README.txt
IRgen optimization opportunities. //===---------------------------------------------------------------------===// The common pattern of -- short x; // or char, etc (x == 10) -- generates an zext/sext of x which can easily be avoided. //===---------------------------------------------------------------------===// Bitfields accesses can be shifted to simplify masking and sign extension. For example, if the bitfield width is 8 and it is appropriately aligned then is is a lot shorter to just load the char directly. //===---------------------------------------------------------------------===// It may be worth avoiding creation of alloca's for formal arguments for the common situation where the argument is never written to or has its address taken. The idea would be to begin generating code by using the argument directly and if its address is taken or it is stored to then generate the alloca and patch up the existing code. In theory, the same optimization could be a win for block local variables as long as the declaration dominates all statements in the block. NOTE: The main case we care about this for is for -O0 -g compile time performance, and in that scenario we will need to emit the alloca anyway currently to emit proper debug info. So this is blocked by being able to emit debug information which refers to an LLVM temporary, not an alloca. //===---------------------------------------------------------------------===// We should try and avoid generating basic blocks which only contain jumps. At -O0, this penalizes us all the way from IRgen (malloc & instruction overhead), all the way down through code generation and assembly time. On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just direct branches! //===---------------------------------------------------------------------===//