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Making use of VFP / NEON floating point multiply-accumulate / subtraction is
difficult on current ARM implementations for a few reasons.
1. Even though a single vmla has latency that is one cycle shorter than a pair
of vmul + vadd, a RAW hazard during the first (4? on Cortex-a8) can cause
additional pipeline stall. So it's frequently better to single codegen
vmul + vadd.
2. A vmla folowed by a vmul, vmadd, or vsub causes the second fp instruction to
stall for 4 cycles. We need to schedule them apart.
3. A vmla followed vmla is a special case. Obvious issuing back to back RAW
vmla + vmla is very bad. But this isn't ideal either:
vmul
vadd
vmla
Instead, we want to expand the second vmla:
vmla
vmul
vadd
Even with the 4 cycle vmul stall, the second sequence is still 2 cycles
faster.
Up to now, isel simply avoid codegen'ing fp vmla / vmls. This works well enough
but it isn't the optimial solution. This patch attempts to make it possible to
use vmla / vmls in cases where it is profitable.
A. Add missing isel predicates which cause vmla to be codegen'ed.
B. Make sure the fmul in (fadd (fmul)) has a single use. We don't want to
compute a fmul and a fmla.
C. Add additional isel checks for vmla, avoid cases where vmla is feeding into
fp instructions (except for the #3 exceptional case).
D. Add ARM hazard recognizer to model the vmla / vmls hazards.
E. Add a special pre-regalloc case to expand vmla / vmls when it's likely the
vmla / vmls will trigger one of the special hazards.
Enable these fp vmlx codegen changes for Cortex-A9.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129775 91177308-0d34-0410-b5e6-96231b3b80d8
105 lines
2.0 KiB
LLVM
105 lines
2.0 KiB
LLVM
; RUN: llc < %s -march=arm -mattr=+vfp2 | FileCheck %s -check-prefix=VFP2
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; RUN: llc < %s -march=arm -mattr=+neon | FileCheck %s -check-prefix=NEON
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; RUN: llc < %s -march=arm -mcpu=cortex-a8 | FileCheck %s -check-prefix=A8
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; RUN: llc < %s -march=arm -mcpu=cortex-a9 | FileCheck %s -check-prefix=A9
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; RUN: llc < %s -mtriple=arm-linux-gnueabi -mcpu=cortex-a9 -float-abi=hard | FileCheck %s -check-prefix=HARD
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define float @t1(float %acc, float %a, float %b) {
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entry:
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; VFP2: t1:
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; VFP2: vmla.f32
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; NEON: t1:
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; NEON: vmla.f32
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; A8: t1:
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; A8: vmul.f32
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; A8: vadd.f32
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%0 = fmul float %a, %b
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%1 = fadd float %acc, %0
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ret float %1
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}
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define double @t2(double %acc, double %a, double %b) {
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entry:
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; VFP2: t2:
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; VFP2: vmla.f64
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; NEON: t2:
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; NEON: vmla.f64
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; A8: t2:
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; A8: vmul.f64
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; A8: vadd.f64
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%0 = fmul double %a, %b
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%1 = fadd double %acc, %0
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ret double %1
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}
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define float @t3(float %acc, float %a, float %b) {
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entry:
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; VFP2: t3:
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; VFP2: vmla.f32
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; NEON: t3:
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; NEON: vmla.f32
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; A8: t3:
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; A8: vmul.f32
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; A8: vadd.f32
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%0 = fmul float %a, %b
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%1 = fadd float %0, %acc
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ret float %1
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}
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; It's possible to make use of fp vmla / vmls on Cortex-A9.
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; rdar://8659675
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define void @t4(float %acc1, float %a, float %b, float %acc2, float %c, float* %P1, float* %P2) {
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entry:
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; A8: t4:
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; A8: vmul.f32
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; A8: vmul.f32
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; A8: vadd.f32
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; A8: vadd.f32
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; Two vmla with now RAW hazard
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; A9: t4:
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; A9: vmla.f32
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; A9: vmla.f32
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; HARD: t4:
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; HARD: vmla.f32 s0, s1, s2
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; HARD: vmla.f32 s3, s1, s4
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%0 = fmul float %a, %b
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%1 = fadd float %acc1, %0
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%2 = fmul float %a, %c
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%3 = fadd float %acc2, %2
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store float %1, float* %P1
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store float %3, float* %P2
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ret void
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}
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define float @t5(float %a, float %b, float %c, float %d, float %e) {
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entry:
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; A8: t5:
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; A8: vmul.f32
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; A8: vmul.f32
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; A8: vadd.f32
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; A8: vadd.f32
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; A9: t5:
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; A9: vmla.f32
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; A9: vmul.f32
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; A9: vadd.f32
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; HARD: t5:
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; HARD: vmla.f32 s4, s0, s1
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; HARD: vmul.f32 s0, s2, s3
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; HARD: vadd.f32 s0, s4, s0
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%0 = fmul float %a, %b
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%1 = fadd float %e, %0
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%2 = fmul float %c, %d
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%3 = fadd float %1, %2
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ret float %3
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}
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