[PATCH] D42401: [ARM] Expand long shifts for Thumb1 to __aeabi_ calls

[Weiming Zhao via Phabricator via llvm-commits]

weimingz added a comment.

On Thumb1, i64 shift is lowered to 20 instructions. For example:
For code like
unsigned long long foo(unsigned long long x, unsigned y) { return x << y;}

clang -mcpu=cortex-m0   -Os  -S generates:

foo:
	.fnstart
@ %bb.0:                                @ %entry
	.save	{r4, r6, r7, lr}
	push	{r4, r6, r7, lr}
	.setfp	r7, sp, #8
	add	r7, sp, #8
	lsls	r1, r2
	movs	r3, #32
	subs	r3, r3, r2
	mov	r4, r0
	lsrs	r4, r3
	orrs	r4, r1
	mov	r3, r2
	subs	r3, #32
	mov	r1, r0
	lsls	r1, r3
	cmp	r3, #0
	bge	.LBB0_2
@ %bb.1:                                @ %entry
	mov	r1, r4
.LBB0_2:                                @ %entry
	lsls	r0, r2
	movs	r2, #0
	cmp	r3, #0
	bge	.LBB0_4
@ %bb.3:                                @ %entry
	mov	r2, r0
.LBB0_4:                                @ %entry
	mov	r0, r2
	pop	{r4, r6, r7, pc}
.Lfunc_end0:

If the i64-shifts are frequently used in source code, the generated code will bloat quickly.


Repository:
  rL LLVM

https://reviews.llvm.org/D42401