Undesirable rematerialization of stack address computation

[eugenis]

Bugzilla Link	36491
Version	trunk
OS	Linux
CC	@echristo,@MatzeB

Extended Description

$ cat 1.c
void use(void *);

void *p;

void f() {
int x;
p = &x;
use(&x);
}

$ bin/clang 1.c -target aarch64-linux-android -O3 -fomit-frame-pointer -c && bin/llvm-objdump -d -r 1.o

   0:	fe 0f 1f f8 	str	x30, [sp, #-16]!
   4:	08 00 00 90 	adrp	x8, #​0
	0000000000000004:  R_AARCH64_ADR_PREL_PG_HI21	p
   8:	e9 33 00 91 	add	x9, sp, #​12
   c:	e0 33 00 91 	add	x0, sp, #​12
  10:	09 01 00 f9 	str	x9, [x8]
	0000000000000010:  R_AARCH64_LDST64_ABS_LO12_NC	p
  14:	00 00 00 94 	bl	#0
	0000000000000014:  R_AARCH64_CALL26	use
  18:	fe 07 41 f8 	ldr	x30, [sp], #​16
  1c:	c0 03 5f d6 	ret

Here x0 and x9 could be merged, but RegisterCoalescer would never consider merging into a physical register. Instead it remats ADDXri %stack.0.x, 0, 0.

This is specific to AArch64 backend. The same does not happen on X86 because ADDXri is asCheapAsAMove, while LEA is not.