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Re: useless cast blocking some optimization in gcc 4.7.3


On Mon, 8 Apr 2013, Laurent Alfonsi wrote:

I have identified a big performance regression between 4.6 and 4.7. (I have enclosed a pathological test).

After investigation, it is because of the += statement applied on 2 signed chars. - It is now type-promoted to "int" when it is written "result += foo()". (since 4.7) - it is type promoted to "unsigned char" when it is written "result = result + foo()".

The "char->int->char" cast is blocking some optimizations in later phases.

Which ones?

Anyway, this doesn't look wrong, so I extended fold optimization in order to catch this case. (patch enclosed)
The patch basically transforms :
(TypeA) ( (TypeB) a1 + (TypeB) a2 ) /* with a1 and a2 of the signed type TypeA */
into :
   a1 + a2

I believe this is legal for any licit a1/a2 input values (no overflow on signed char).

I don't think this is ok, please refer to the discussion around the PR and patch that added this conversion, it was done on purpose.

According to this (4th item)

char a=100;

is perfectly defined and a is -56 (assuming a signed 8 bit char and a strictly larger int). However, your transformation turns it into undefined behavior: an addition that overflows in a type with TYPE_OVERFLOW_UNDEFINED.

Marc Glisse

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