[llvm-commits] [ARM] Fix for blocking PR13790. AAPCS byval params issue.

[Stepan Dyatkovskiy]

Hi James,

>    * Why the need to change the LDRB_POST_IMM-Crash.ll testcase so
> drastically? (changing from doubles to i8s)

For non EABI mode COPY_STRUCT_BYVAL is expanded onto set of ldr/str (or 
vld1/vst1 for EABI) instructions,
But if there are some remainder, less than copy-unit-size (for vld1 it 
is 8 bytes) it copied with ldrb/strb instructions (see 
ARMTargetLowering::EmitStructByval for more details).

Current test uses (i32, { double, double, double } ) params set. Params 
are passed with next way:
1. i32 -> r0
2. First three 32 bit words of struct -> r1,r2,r3 (12 bytes)
    Next 56 bytes copied with 7 vld1/vst1 instructions.
    Remainder (4 bytes) copied with ldrb/strb instructions.

But for AAPCS mode we shouldn't use r1,r2,r3 set for head part of 
structure. We can use r2,r3 only. So bug-fix changes output code for 
this test. After this patch params will be passed with next way:
1. i32 ->r0
2. First *two* 32 bit words of struct -> r2,r3 (8 bytes)
    Next 64 bytes are copied with 8 vld1/vst1 instructions.
    No bytes remained. No remainder => no ldrb instructions.

IMHO the best way to get llvm to emit ldrb instructions is to form 1 
byte aligned structure and then to get it to emit strictly 1 byte into 
memory.

I changed param set for function to (i32, i32, i32, { i8 <-- 5 times }).
It get llvm to pass params with next way:
1. i32 -> r0
2. i32 -> r1
3. i32 -> r2
4. i8 i8 i8 i8 -> r3
    We have one byte remained -> emit ldrb/strb

Note, it works for any mode (EABI/non-EABI), since biggest field in 
structure is 8 bit length, that makes it 1 byte aligned.

And... Current test contains some extra nodes: memcpy, alloca, etc 
(since it was made with bugpoint and I cleaned it then inaccurately). 
And when I was writing that test I wasn't pretty familiar with this part 
of codegen. Now I decided to make it from scratch: as simplest as it 
possible.

>    * if (Subtarget->isAAPCS_ABI()  <-- surely this should be enabled when
> using the aapcs-vfp ABI too?
aapcs-vfp = AAPCS mode + enable VFP extension, right? The answer is 
"yes" then. How to check that aapcs-vfp is used?

-Stepan.