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add some notes on compares + binops. Remove redundant entries.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125702 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2011-02-17 01:43:46 +00:00
parent de1d8a544c
commit cb40195641
1 changed files with 50 additions and 23 deletions
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73
lib/Target/README.txt
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@ -873,6 +873,12 @@ rshift_gt (unsigned int a)
if ((a >> 2) > 5)
bar ();
}
void neg_eq_cst(unsigned int a) {
if (-a == 123)
bar();
}
All should simplify to a single comparison. All of these are
currently not optimized with "clang -emit-llvm-bc | opt
-std-compile-opts".
@ -2219,31 +2225,52 @@ avoids partial register stalls in some important cases.
//===---------------------------------------------------------------------===//
Some missed instcombine xforms (from GCC PR14753):
We should fold compares like this:
void bar (void);
%1266 = add nsw i32 %.84.i.i.i, 1
%560 = add nsw i32 %556, 1
%1267 = icmp slt i32 %1266, %560
void mask_gt (unsigned int a) {
/* This is equivalent to a > 15. */
if ((a & ~7) > 8)
bar();
}
void neg_eq_cst(unsigned int a) {
if (-a == 123)
bar();
}
void minus_cst(unsigned int a) {
if (20 - a == 5)
bar();
}
void rotate_cst (unsigned a) {
a = (a << 10) | (a >> 22);
if (a == 123)
bar ();
}
to a single 'icmp slt' when the add's have a single use, since they are NSW.
//===---------------------------------------------------------------------===//
We don't fold (icmp (add) (add)) unless the two adds only have a single use.
There are a lot of cases that we're refusing to fold in (e.g.) 256.bzip2, for
example:
%indvar.next90 = add i64 %indvar89, 1 ;; Has 2 uses
%tmp96 = add i64 %tmp95, 1 ;; Has 1 use
%exitcond97 = icmp eq i64 %indvar.next90, %tmp96
We don't fold this because we don't want to introduce an overlapped live range
of the ivar. However if we can make this more aggressive without causing
performance issues in two ways:
1. If *either* the LHS or RHS has a single use, we can definitely do the
transformation. In the overlapping liverange case we're trading one register
use for one fewer operation, which is a reasonable trade. Before doing this
we should verify that the llc output actually shrinks for some benchmarks.
2. If both ops have multiple uses, we can still fold it if the operations are
both sinkable to *after* the icmp (e.g. in a subsequent block) which doesn't
increase register pressure.
There are a ton of icmp's we aren't simplifying because of the reg pressure
concern. Care is warranted here though because many of these are induction
variables and other cases that matter a lot to performance, like the above.
Here's a blob of code that you can drop into the bottom of visitICmp to see some
missed cases:
{ Value *A, *B, *C, *D;
if (match(Op0, m_Add(m_Value(A), m_Value(B))) &&
match(Op1, m_Add(m_Value(C), m_Value(D))) &&
(A == C || A == D || B == C || B == D)) {
errs() << "OP0 = " << *Op0 << " U=" << Op0->getNumUses() << "\n";
errs() << "OP1 = " << *Op1 << " U=" << Op1->getNumUses() << "\n";
errs() << "CMP = " << I << "\n\n";
}
}
//===---------------------------------------------------------------------===//