https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77568
Bug ID: 77568
Summary: [7 regression] CSE/PRE/Hoisting blocks common
instruction contractions
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: wdijkstr at arm dot com
Target Milestone: ---
The recently introduced code hoisting aggressively moves common subexpressions
that might otherwise be mergeable with other operations. This caused a large
regression in one benchmark. A simple reduced test shows the issue:
float f(float x, float y, float z, int a)
{
if (a > 100)
x += y * z;
else
x -= y * z;
return x;
}
This now produces on AArch64:
f:
fmul s2, s1, s2
cmp w0, 100
fadd s1, s0, s2
fsub s0, s0, s2
fcsel s0, s0, s1, le
ret
Note the issue is not limited to hoisting, CSE/PRE cause similar issues:
void g(int, int);
int f2(int x)
{
g(x, x+1);
g(x, x+1);
return x+1;
}
f2:
stp x29, x30, [sp, -32]!
add x29, sp, 0
stp x19, x20, [sp, 16]
add w19, w0, 1
mov w20, w0
mov w1, w19
bl g
mov w1, w19
mov w0, w20
bl g
mov w0, w19
ldp x19, x20, [sp, 16]
ldp x29, x30, [sp], 32
ret
Given x+1 is used as a function argument, there is no benefit in making it
available as a CSE after each call - repeating the addition is cheaper than
using an extra callee-save and copying it several times.
This shows a similar issue for bit tests. Most targets support ANDS or bit test
as a single instruction (or even bit test+branch), so CSEing the (x & C)
actually makes things worse:
void f3(char *p, int x)
{
if (x & 1) p[0] = 0;
if (x & 2) p[1] = 0;
if (x & 4) p[2] = 0;
if (x & 8) p[2] = 0;
g(0,0);
if (x & 1) p[3] = 0;
if (x & 2) p[4] = 0;
if (x & 4) p[5] = 0;
if (x & 8) p[6] = 0;
}
This uses 4 callee-saves to hold the (x & C) CSEs. Doing several such bit tests
in a more complex function means you quickly run out of registers...
Given it would be much harder to undo these CSEs at RTL level (combine does
most contractions but can only do it intra-block), would it be reasonable to
block CSEs for these special cases?
