diff options
Diffstat (limited to 'llvm/docs/SourceLevelDebugging.rst')
| -rw-r--r-- | llvm/docs/SourceLevelDebugging.rst | 17 |
1 files changed, 13 insertions, 4 deletions
diff --git a/llvm/docs/SourceLevelDebugging.rst b/llvm/docs/SourceLevelDebugging.rst index e7f8b13b589..7930d2cb6cc 100644 --- a/llvm/docs/SourceLevelDebugging.rst +++ b/llvm/docs/SourceLevelDebugging.rst @@ -531,6 +531,7 @@ within the LLVM IR. By the end of CodeGen, this becomes a mapping from each variable to their machine locations over ranges of instructions. From IR to object emission, the major transformations which affect variable location fidelity are: + 1. Instruction Selection 2. Register allocation 3. Block layout @@ -539,6 +540,14 @@ each of which are discussed below. In addition, instruction scheduling can significantly change the ordering of the program, and occurs in a number of different passes. +Some variable locations are not transformed during CodeGen. Stack locations +specified by ``llvm.dbg.declare`` are valid and unchanging for the entire +duration of the function, and are recorded in a simple MachineFunction table. +Location changes in the prologue and epilogue of a function are also ignored: +frame setup and destruction may take several instructions, require a +disproportionate amount of debugging information in the output binary to +describe, and should be stepped over by debuggers anyway. + Variable locations in Instruction Selection and MIR --------------------------------------------------- @@ -573,10 +582,10 @@ inserted. These ``DBG_VALUE`` instructions appear thus: DBG_VALUE %1, $noreg, !123, !DIExpression() And have the following operands: - * The first operand can record the variable location as a register, an - immediate, or the base address register if the original debug intrinsic - referred to memory. ``$noreg`` indicates the variable location is undefined, - equivalent to an ``undef`` dbg.value operand. + * The first operand can record the variable location as a register, + a frame index, an immediate, or the base address register if the original + debug intrinsic referred to memory. ``$noreg`` indicates the variable + location is undefined, equivalent to an ``undef`` dbg.value operand. * The type of the second operand indicates whether the variable location is directly referred to by the DBG_VALUE, or whether it is indirect. The ``$noreg`` register signifies the former, an immediate operand (0) the |
