Re: [PATCH] Fold sign of LSHIFT_EXPR to eliminate no-op conversions.

[Jeff Law via Gcc-patches]

On 8/22/2021 6:25 PM, Roger Sayle wrote:
This short patch teaches fold that it is "safe" to change the sign
of a left shift, to reduce the number of type conversions in gimple.
As an example:

unsigned int foo(unsigned int i) {
   return (int)i << 8;
}

is currently optimized to:

unsigned int foo (unsigned int i)
{
   int i.0_1;
   int _2;
   unsigned int _4;

   <bb 2> [local count: 1073741824]:
   i.0_1 = (int) i_3(D);
   _2 = i.0_1 << 8;
   _4 = (unsigned int) _2;
   return _4;
}

with this patch, this now becomes:

unsigned int foo (unsigned int i)
{
   unsigned int _2;

   <bb 2> [local count: 1073741824]:
   _2 = i_1(D) << 8;
   return _2;
}

which generates exactly the same assembly language.  Aside from the
reduced memory usage, the real benefit is that no-op conversions tend
to interfere with many folding optimizations.  For example,

unsigned int bar(unsigned char i) {
     return (i ^ (i<<16)) | (i<<8);
}

currently gets (tangled in conversions and) optimized to:

unsigned int bar (unsigned char i)
{
   unsigned int _1;
   unsigned int _2;
   int _3;
   int _4;
   unsigned int _6;
   unsigned int _8;

   <bb 2> [local count: 1073741824]:
   _1 = (unsigned int) i_5(D);
   _2 = _1 * 65537;
   _3 = (int) i_5(D);
   _4 = _3 << 8;
   _8 = (unsigned int) _4;
   _6 = _2 | _8;
   return _6;
}

but with this patch, bar now optimizes down to:

unsigned int bar(unsigned char i)
{
   unsigned int _1;
   unsigned int _4;

   <bb 2> [local count: 1073741824]:
   _1 = (unsigned int) i_3(D);
   _4 = _1 * 65793;
   return _4;

}


This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures.  Ok for mainline?


2021-08-23  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        * match.pd (shift transformations): Change the sign of an
        LSHIFT_EXPR if it reduces the number of explicit conversions.

gcc/testsuite/ChangeLog
        * gcc.dg/fold-convlshift-1.c: New test case.
        * gcc.dg/fold-convlshift-2.c: New test case.
Presumably we're relying on the fact that the type of the convert:@1 has 
to be the same type as @0, thus there's no need to check anything 
related to @1.

OK
jeff