Re: [Qemu-devel] [PATCH] RFC: TCG constant propagation.

[Filip Navara <filip.navara@gmail.com>]

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On Thu, Jul 23, 2009 at 11:25 AM, Laurent
Desnogues<laurent.desnogues@gmail.com> wrote:
> On Thu, Jul 23, 2009 at 11:08 AM, Filip Navara<filip.navara@gmail.com> wrote:
>> Also it looks that the actual code size is worse for TCG ARM target
>> with this optimization. To mitigate it I propose the other attached
>> patch which uses the barrel shifter for more optimal generation of the
>> "movi" operation. I can't test it, so any help is welcome.
>
> This is wrong for two reasons:
>   - this is a rotation, you only check for shifts

I deliberately didn't do it, but it's surely an opportunity for
further optimization.

>   - the rotation amount is multiplied by 2 before being applied to the
>     8-bit immediate field.

As Paul already pointed out, that's compensated by the shift (<< 7
instead of << 8) and the "& ~1" after ctz32 call.

Attached is fixed version of the patch, I forgot one "return" there.

Best regards,
Filip Navara

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From d326aabe85412a91c17e0be2ebbfd078bec16713 Mon Sep 17 00:00:00 2001
From: Filip Navara <filip.navara@gmail.com>
Date: Thu, 23 Jul 2009 11:01:32 +0200
Subject: [PATCH 2/2] Use ARM barrel shifter for more optimal generation of MVN, MOV instructions for the MOVI TCG operation.

---
 tcg/arm/tcg-target.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/tcg/arm/tcg-target.c b/tcg/arm/tcg-target.c
index 7ef2b89..9748ce2 100644
--- a/tcg/arm/tcg-target.c
+++ b/tcg/arm/tcg-target.c
@@ -334,6 +334,7 @@ static inline void tcg_out_movi32(TCGContext *s,
                 int cond, int rd, int32_t arg)
 {
     int offset = (uint32_t) arg - ((uint32_t) s->code_ptr + 8);
+    int shift;
 
     /* TODO: This is very suboptimal, we can easily have a constant
      * pool somewhere after all the instructions.  */
@@ -346,6 +347,17 @@ static inline void tcg_out_movi32(TCGContext *s,
                 tcg_out_dat_imm(s, cond, ARITH_ADD, rd, 15, offset) :
                 tcg_out_dat_imm(s, cond, ARITH_SUB, rd, 15, -offset);
 
+    if (arg < 0x100)
+        return tcg_out_dat_imm(s, cond, ARITH_MOV, rd, 0, arg & 0xff);
+
+    shift = ctz32(arg) & ~1;
+    if (!((arg >> shift) & ~0xff))
+        return tcg_out_dat_imm(s, cond, ARITH_MOV, rd, 0, (arg >> shift) + (shift << 7));
+
+    shift = ctz32(~arg) & ~1;
+    if (!((~arg >> shift) & ~0xff))
+        return tcg_out_dat_imm(s, cond, ARITH_MVN, rd, 0, (~arg >> shift) + (shift << 7));
+
 #ifdef __ARM_ARCH_7A__
     /* use movw/movt */
     /* movw */
-- 
1.6.3.2.1299.gee46c