On Tue, 3 Jun 2014, Richard Sandiford wrote: > Sandra Loosemore <san...@codesourcery.com> writes: > > Catherine included an earlier version of this patch with the microMIPS > > submission a couple years ago: > > > > https://gcc.gnu.org/ml/gcc-patches/2012-07/msg00972.html > > > > Richard's response was: > > > >> Looks like the wrong place to do this. Please treat this as a separate > >> patch and get a tree expert to comment. > > > > So, here is the separate patch, finally. :-) Can a tree expert > > comment? I dug around and didn't see another obvious hook to set > > function alignment in a target-specific way depending on attributes of > > the function. > > > > Besides the new test cases, I regression-tested this on mips-sde-elf > > using Mentor's usual assortment of multilibs, specifically including one > > for microMIPS. > > OK, I asked Richi on IRC today. To recap, one of the things I was > worried about was a test against the address, like (foo & 2) == 0, > being optimised away before we set the alignment. Richi pointed > out that my idea downthread about cgraph_create_node would also be > too late to avoid that. Also, looking at it now, I see that we don't > trust DECL_ALIGN on functions anyway. From get_object_alignment_2: > > /* Extract alignment information from the innermost object and > possibly adjust bitpos and offset. */ > if (TREE_CODE (exp) == FUNCTION_DECL) > { > /* Function addresses can encode extra information besides their > alignment. However, if TARGET_PTRMEMFUNC_VBIT_LOCATION > allows the low bit to be used as a virtual bit, we know > that the address itself must be at least 2-byte aligned. */ > if (TARGET_PTRMEMFUNC_VBIT_LOCATION == ptrmemfunc_vbit_in_pfn) > align = 2 * BITS_PER_UNIT; > } > > And since we use the low bit to encode the ISA mode on MIPS, the upshot > is that we never assume any alignment for functions. So there's nothing > to worry about after all. > > Richi suggested just changing the alignment at output time. I assume > that would be a case of replacing the DECL_ALIGN in: > > /* Tell assembler to move to target machine's alignment for functions. */ > align = floor_log2 (DECL_ALIGN (decl) / BITS_PER_UNIT); > if (align > 0) > { > ASM_OUTPUT_ALIGN (asm_out_file, align); > } > > with a hook. (Is that right?)
Yeah, kind of. Of course if DECL_ALIGN on function-decls is "unused" then we may as well initialize it to 1 in tree.c and at an appropriate stage adjust it to the result of a target hook invocation. Appropriate stage would be the above place (which means DECL_ALIGN is essentially "unused" for FUNCTION_DECLs). So ... can you massage the DECL_ALIGN macro to ICE on FUNCTION_DECLs and see where we access it? (generic code will possibly trip on it, but the question is is there any user that cares?) Richard.
