Re: [PATCH, ARM] stop changing signedness in PROMOTE_MODE

[Richard Biener <richard.guenther@gmail.com>]

On July 14, 2015 6:49:18 PM GMT+02:00, Jim Wilson <jim.wilson@linaro.org> wrote:
>On Tue, Jul 14, 2015 at 9:13 AM, Richard Earnshaw
><Richard.Earnshaw@foss.arm.com> wrote:
>> We went through this a couple of weeks back.  The backend
>documentation
>> for PROMOTE_MODE says:
>
>I disagree that this is a fully resolved issue.  I see clear problems
>with how the ARM port uses PROMOTE_MODE.
>
>> " For most machines, the macro definition does not change
>@var{unsignedp}.
>> However, some machines, have instructions that preferentially handle
>> either signed or unsigned quantities of certain modes.  For example,
>on
>> the DEC Alpha, 32-bit loads from memory and 32-bit add instructions
>> sign-extend the result to 64 bits.  On such machines, set
>> @var{unsignedp} according to which kind of extension is more
>efficient."
>
>The Alpha case is different than the ARM case.  The Alpha only changes
>sign for 32-bit values in 64-bit registers.  The alpha happens to have
>a nice set of instructions that operates on 32-bit values, that
>accepts these wrong-signed values and handle them correctly.  Thus on
>the alpha, it appears that there are no extra zero/sign extends
>required, and everything is the same speed or faster with the wrong
>sign.  The MIPS port works the same way.  This is not true on the arm
>though.  The arm port is changing sign on 8 and 16 bit value, but does
>not have instructions that directly operate on 8 and 16 bit values.
>This requires the compiler to emit extra zero/sign extend instructions
>that affect the performance of the code.  Other than the thumb1 8-bit
>load, and the pre-armv6 use of andi for 8-bit zero-extend, I haven't
>seen anything that is faster in the wrong sign, and there are a number
>of operations that are actually slower because of the extra zero/sign
>extends required by the arm PROMOTE_MODE definition.  I've given some
>examples.
>
>I have since noticed that the relatively new pass to optimize away
>unnecessary zero/sign extensions is actually fixing some of the damage
>caused by the ARM PROMOTE_MODE definition.  But there are still some
>cases that won't get fixed, as per my earlier examples.  It is better
>to emit the fast code at the beginning than to rely on an optimization
>pass to convert the slow code into fast code.  So I think the ARM
>PROMOTE_MODE definition still needs to be fixed.
>
>> So clearly it anticipates that all permitted extensions should work,
>and
>> in particular it makes no mention of this having to match some
>> abi-mandated promotions.  That makes this a MI bug not a target one.
>
>If you look at older gcc releases, like gcc-2.95.3, you will see that
>there is only PROMOTE_MODE and a boolean PROMOTE_FUNCTION_ARGS which
>says whether PROMOTE_MODE is applied to function arguments or not.
>You can't have different zero/sign extensions for parameters and
>locals in gcc-2.95.3.  The fact that you can have it today is more an
>accident than a deliberate choice.  When PROMOTE_FUNCTION_ARGS was
>hookized, it was made into a separate function, and for the ARM port,
>the code for PROMOTE_MODE was not correctly copied into the new hook,
>resulting in the accidental difference for parameter and local
>zero/sign promotion for ARM.  The PROMOTE_MODE docs were written back
>when different sign/zero extensions for parms/locals weren't possible,
>and hence did not consider the case.
>
>Now that we do have the problem, we can't fix it without an ARM port
>ABI change, which is undesirable, so we may have to fix it with a MI
>change.
>
>There were two MI changes suggested, one was fixing the out-of-ssa
>pass to handle SUBREG_PROMOTED_P promotions.  The other was to
>disallow creating PHI nodes between parms and locals.  I haven't had a
>chance to try implementing the second one yet; I hope to work on that
>today.

I will definitely veto such restriction.

Richard.

>Jim