What does a dangerous relocation error mean?

问题

I am getting a linking error:

dangerous relocation: l32r: Literal placed after use:

I am still trying to debug; however, I want to better understand this error. I understand what relocation is; however, I am not sure how it can be dangerous and was looking for some clarification. Also, a small code snippet that could generate this type of error would be helpful.

In short, what is "a dangerous relocation"?

回答1:

This is a two-part answer, as there are really two questions here, one general ("what's a dangerous relocation?") and one specific to the Xtensa ("why can't you have a literal placed after where it's used in the code?").

What's all this dangerous relocation stuff about, anyway?

To understand what a 'dangerous relocation' is, we must first understand what a relocation is. As a compiler is generating an object file from some piece of code, it will need to reference symbols that are defined somewhere else: perhaps in another object file in the link, or perhaps in a shared library. However, the compiler does not know the addresses of external symbols when compiling a given object file. It must emit a relocation to serve as a named placeholder, telling the linker "OK, shove the address of foobar into this spot, and oh, you have to do X, Y, and Z to it to make it fit into the instructions there."

Most of the time, this works without a hitch, you get a binary out of your linker, and Bob's your uncle. When this process breaks down, and the linker cannot make the address of the symbol the compiler gave it fit into the instructions at the site of the relocation, it gives up and tosses out a 'dangerous relocation' message (among others -- the all-too-common 'relocation truncated to fit' pops out of this process as well) to inform the programmer that something has gone terribly wrong.

What's wrong with a literal placed after where it's used?

Now that we know what a generic 'dangerous relocation' is, we can move on to the second half of the error message, namely "l32r: Literal placed after use". The Xtensa uses an instruction known as L32R to load constant values from memory that don't fit into the Xtensa's MOVI immediate load instruction, which has a 12-bit signed immediate field. The L32R instruction is described in the Xtensa ISA reference as follows:

L32R is a PC-relative 32-bit load from memory. It is typically used to load constant values into a register when the constant cannot be encoded in a MOVI instruction.

L32R forms a virtual address by adding the 16-bit one-extended constant value encoded in the instruction word shifted left by two to the address of the L32R plus three with the two least significant bits cleared. Therefore, the offset can always specify 32-bit aligned addresses from -262141 to -4 bytes from the address of the L32R instruction. 32 bits (four bytes) are read from the physical address. This data is then written to address register at.

Given the restrictions on L32R quoted above, the error message breaks down quite nicely: the compiler generated a L32R to load a constant (which could be a value or an address) somewhere in your code, but either the constant's value was not available to the compiler (think extern const), or the address needed to be filled in by the linker (this is the likely case). So, it emitted this L32R relocation to tell the linker to 'fill in the blank' in the L32R instruction with the address of a constant value or constant address somewhere in your program. However, the linker couldn't find anywhere in the previous 256KB of code -- or literal pool, depending on how your compiler and Xtensa core are configured -- to shove a constant, so it gave up and spat out the error message you asked about.

How does one fix this?

Unfortunately, a 'dangerous relocation' of this sort depends on code size, so unless you have a bona fide compiler or linker bug on your hands, reproducing it with a small snippet of code will be impossible. There are two possible causes you can try to address, though.

There's no room for my literal pool!

If you are compiling with -mno-text-section-literals (which is the default), the linker gets fed the literal pools as separate sections which it then has to interleave with the code sections. If you have a particularly large object file in your link, it may have over 256KB of code in its .text section, leaving nowhere in the range of a L32R instruction for the linker to place the associated literal pool section at. Compiling with -mtext-section-literals should eliminate the error; if it does not work, you have that flag on already, or if you are using -ffunction-sections (which places each function into its own section; it is sometimes used in embedded work to allow the linker to throw out unused code), read on.

The linker (or assembler) still can't find a place to put my literals!

When the compiler and assembler are told to emit literals into the text section, they restrict placement of the literal pools to before the functions that use them (i.e. before the ENTRY instruction of the function) in order to minimize the risk that the literal pools will be executed as code, with obviously bad results. If you have an extremely long function in your code -- I shudder to think what sort of function could generate more than 256KB of code -- the 'default' literal pool placed before the ENTRY instruction can wind up out of range of L32R instructions near the end of the function. Normally, the compiler will emit an assembler directive known as .literal_position, as well as a jump around the mid-function literal pool, to provide the assembler and linker with an extra place to shove literals into. You can tell the compiler to output an assembler listing using -save-temps and then search it for .literal_position directives; if one isn't present in a function that has L32R instructions past the 256KB mark, congratulations! You just found a compiler bug!

What else could happen to produce this?

The only other circumstance I see that can provoke such a problem is if there is nowhere before the ENTRY instruction that the compiler or linker can put a literal pool, and the compiler can't figure this out on its own -- this can occur with interrupt handlers, or functions that are explicitly placed at the beginning of a physical memory boundary by the linker script. In this case, you will need to insert the .literal_position directive and its associated jump & label by hand in an asm statement at the top of the culprit function in order to provide the assembler with a place to put the culprit function's literals. As the GAS manual puts it:

The assembler will automatically place text section literal pools before ENTRY instructions, so the .literal_position directive is only needed to specify some other location for a literal pool. You may need to add an explicit jump instruction to skip over an inline literal pool.

For example, an interrupt vector does not begin with an ENTRY instruction so the assembler will be unable to automatically find a good place to put a literal pool. Moreover, the code for the interrupt vector must be at a specific starting address, so the literal pool cannot come before the start of the code. The literal pool for the vector must be explicitly positioned in the middle of the vector (before any uses of the literals, due to the negative offsets used by PC-relative L32R instructions).

Wait, I'm using the absolute literal option!

If you have the LITBASE option enabled in your Xtensa core and are getting this error, this is a sign that your literal pool has overflowed. The compiler should generate the 'glue' needed to switch literal pools in this case, though: if it doesn't, congratulations! You have just found a compiler bug!