Patchwork [ARM] Fix a latent issue in arm_print_operand

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Submitter Ramana Radhakrishnan
Date Dec. 12, 2011, 8:10 p.m.
Message ID <CACUk7=X912WSUxH8UMrzT7syTVCLbXqhScQLAdifDBpgAnj7QA@mail.gmail.com>
Download mbox | patch
Permalink /patch/130812/
State New
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Comments

Ramana Radhakrishnan - Dec. 12, 2011, 8:10 p.m.
Hi,

This fixes a latent issue in arm_print_operand where we would have
generated a 128 bit alignment hint for any neon instruction that used
the %A notation for printing memory addresses.

Given that you can never have the load of a 64 bit quantity with an
128 bit alignment (indeed the list of instructions that I could think
of that we might generate for this purpose is as below.)


   vld1.8 d0, [r0]
   vld1.16 d0, [r0]
   vld1.32 d0, [r0]
   vld1.64 d0, [r0]

   vld2.32           {d0[0], d1[0]}, [r0]
   vld4.16           {d0[0], d1[0], d2[0], D3[0]}, [r0]

This is not an issue today for the following reasons :

From the list above:

1. The first 4 instructions will not come out of the compiler today
(we put out vldr's or vldmia) instructions for auto-vectorized code.
Indeed I found this only when I replaced the vldmia's and the vldr's
with vld1's.

2. The last 2 instructions only come out of intrinsics. The first
   4 will also get generated with the intrinsics but .....
   we get away with this because of the type punning in arm_neon.h
   Even in the event we use this in that form, I doubt
   if we'll ever change that situation.

I was able to trigger this when I tried to get the backend to generate
vld1.64 instructions for appropriate vector modes instead of vldmia
/ vstmia in little endian configurations. (That will follow once
testing completes)


Ok ?

cheers
Ramana


2011-12-08  Ramana Radhakrishnan  <ramana.radhakrishnan@linaro.org>

        * config/arm/arm.c (arm_print_operand): Only allow hints
        of 128 bits for 16 byte loads.

Patch

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index d1d0c22..bfdac51 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -17654,8 +17654,8 @@  arm_print_operand (FILE *stream, rtx x, int code)
        /* Only certain alignment specifiers are supported by the hardware.  */
        if (memsize == 16 && (align % 32) == 0)
          align_bits = 256;
-       else if ((memsize == 8 || memsize == 16) && (align % 16) == 0)
-         align_bits = 128;
+       else if ((memsize == 16) && (align % 16) == 0)
+           align_bits = 128;
        else if ((align % 8) == 0)
          align_bits = 64;
        else