https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92979
Bug ID: 92979
Summary: bswap not finding a bswap with a memory load at the
beginging of the instruction stream
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pinskia at gcc dot gnu.org
Target Milestone: ---
Target: aarch64-linux-gnu
Take these two functions:
unsigned g(unsigned *a)
{
unsigned M0 = *a & 0xff;
unsigned M1 = (*a>>8) & 0xff;
unsigned M2 = (*a>>16) & 0xff;
unsigned M3 = (*a>>24) & 0xff;
unsigned t = 0;
t |= M0;
t <<= 8;
t |= M1;
t <<= 8;
t |= M2;
t <<= 8;
t |= M3;
return t;
}
unsigned g1(unsigned a)
{
unsigned M0 = a & 0xff;
unsigned M1 = (a>>8) & 0xff;
unsigned M2 = (a>>16) & 0xff;
unsigned M3 = (a>>24) & 0xff;
unsigned t = 0;
t |= M0;
t <<= 8;
t |= M1;
t <<= 8;
t |= M2;
t <<= 8;
t |= M3;
return t;
}
----- CUT ---
Only g1 is detected as bswap while g is not. The problem is the way bswap is
too "agressive" in looking through the instruction stream to find loads.
I found this while implementing lowering of bit-fields.
