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Making use of VFP / NEON floating point multiply-accumulate / subtraction is

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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.

Work in progress, only A+B are enabled.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120960 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Evan Cheng
2010-12-05 22:04:16 +00:00
parent 0a3fdd6e11
commit 48575f6ea7
20 changed files with 773 additions and 192 deletions
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3
test/CodeGen/ARM/reg_sequence.ll
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@@ -270,8 +270,9 @@ define arm_aapcs_vfpcc float @t9(%0* nocapture, %3* nocapture) nounwind {
define arm_aapcs_vfpcc i32 @t10() nounwind {
entry:
; CHECK: t10:
; CHECK: vmul.f32 q8, q8, d0[0]
; CHECK: vmov.i32 q9, #0x3F000000
; CHECK: vmla.f32 q8, q8, d0[0]
; CHECK: vadd.f32 q8, q8, q8
%0 = shufflevector <4 x float> zeroinitializer, <4 x float> undef, <4 x i32> zeroinitializer ; <<4 x float>> [#uses=1]
%1 = insertelement <4 x float> %0, float undef, i32 1 ; <<4 x float>> [#uses=1]
%2 = insertelement <4 x float> %1, float undef, i32 2 ; <<4 x float>> [#uses=1]