https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97784
Bug ID: 97784
Summary: Expressions evaluated as long chain instead of as tree
or the like
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: segher at gcc dot gnu.org
Target Milestone: ---
When compiling something like
#define O +
long x4(long x, long a, long b, long c, long d) { return x O a O b O c O d; }
we end up with machine code like
add 3,3,4 # 10 [c=4 l=4] *adddi3/0
add 3,3,5 # 11 [c=4 l=4] *adddi3/0
add 3,3,6 # 12 [c=4 l=4] *adddi3/0
add 3,3,7 # 18 [c=4 l=4] *adddi3/0
blr # 30 [c=4 l=4] simple_return
Every of those "add" insns depends on the result of the previous one,
making this slower than necessary: it has the latency of 4 add insns in
series, while some can be done in parallel.
This problem is there on gimple level already:
_1 = x_4(D) + a_5(D);
_2 = _1 + b_6(D);
_3 = _2 + c_7(D);
_9 = _3 + d_8(D);
return _9;
A very similar problem also happens as a result of RTL unrolling.
