[PATCH] Use broadcasts to optimize overall size when loading constant splat vectors (x86-64 with AVX or AVX2)

[Demikhovsky, Elena]

Ok, if you want to use VMOVDDUP, you still can do it via pattern in td file. This pattern works perfect:

--- lib/Target/X86/X86InstrSSE.td       (revision 217862)
+++ lib/Target/X86/X86InstrSSE.td       (working copy)
@@ -5279,6 +5279,11 @@
                              (v2i64 (scalar_to_vector (loadi64 addr:$src))))),
             (VMOVDDUPrm addr:$src)>, Requires<[HasAVX]>;
 
+  def : Pat<(v2f64 (X86VBroadcast (loadf64 addr:$src))),
+            (VMOVDDUPrm addr:$src)>;
+  def : Pat<(v2i64 (X86VBroadcast (loadi64 addr:$src))),
+            (VMOVDDUPrm addr:$src)>;
+
   // 256-bit version
   def : Pat<(X86Movddup (loadv4f64 addr:$src)),
             (VMOVDDUPYrm addr:$src)>;

-  Elena


-----Original Message-----
From: Sanjay Patel [mailto:spatel at rotateright.com] 
Sent: Tuesday, September 16, 2014 18:50
To: spatel at rotateright.com; nrotem at apple.com; chandlerc at gmail.com; Andrea_DiBiagio at sn.scee.net; Demikhovsky, Elena
Cc: llvm-commits at cs.uiuc.edu
Subject: Re: [PATCH] Use broadcasts to optimize overall size when loading constant splat vectors (x86-64 with AVX or AVX2)

>>! In D5347#10, @delena wrote:
> I just suggest to add this pattern to X86InstrSSE.td:
> 
> def : Pat<(v2i64 (X86VBroadcast (loadi64 addr:$src))),
>           (v2i64 (EXTRACT_SUBREG (v4i64 (VBROADCASTSDYrm addr:$src)),sub_xmm)))>;

I tried this, but it's not producing the codegen that I want. Specifically, we want to use movddup when possible, and we don't want to alter codegen at all when not optimizing for size. (Apologies for pattern ignorance - I haven't used these yet.)

1. In the testcase for v2f64, no splat is generated (movddup expected).

2. In the testcase for v2i64 with AVX, we get:
   vbroadcastsd	LCPI4_0(%rip), %ymm1
   vpaddq	%xmm1, %xmm0, %xmm0
   vzeroupper  <--- can the pattern be rewritten to avoid this? even if yes, movddup is smaller than broadcastsd

This is worse in size than what my patch produces:
   vmovddup	LCPI4_0(%rip), %xmm1
   vpaddq	%xmm1, %xmm0, %xmm0

3. In the testcase for v4i64 with AVX, we again would generate vbroadcastsd
   vbroadcastsd	LCPI5_0(%rip), %ymm1
   vextractf128	$1, %ymm0, %xmm2
   vpaddq	%xmm1, %xmm2, %xmm2
   vpaddq	%xmm1, %xmm0, %xmm0
   vinsertf128	$1, %xmm2, %ymm0, %ymm0

But movddup is better because it is one byte smaller than vbroadcastsd.

4. Using the pattern also caused a failure in test/CodeGen/X86/exedepsfix-broadcast.ll because a broadcast is generated even when not optimizing for size. I don't think we want to use a broadcast in that case?

http://reviews.llvm.org/D5347


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