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* better heuristics to decide when a mul by a constant shall be replaced by shift/add/sub sequences

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git-svn-id: trunk@22300 -
This commit is contained in:
florian 2012-09-03 13:39:52 +00:00
parent 2f8027c63f
commit 59012afe26
2 changed files with 52 additions and 3 deletions
compiler/arm
cgcpu.pas
tests/test
tmul1.pp

30
compiler/arm/cgcpu.pas
View File

@ -665,7 +665,9 @@ unit cgcpu;
bitsset : byte;
negative : boolean;
first : boolean;
b,
cycles : byte;
maxeffort : byte;
begin
result:=true;
cycles:=0;
@ -677,6 +679,28 @@ unit cgcpu;
multiplier:=dword(abs(a));
bitsset:=popcnt(multiplier and $fffffffe);
{ heuristics to estimate how much instructions are reasonable to replace the mul,
this is currently based on XScale timings }
{ in the simplest case, we need a mov to load the constant and a mul to carry out the
actual multiplication, this requires min. 1+4 cycles
because the first shift imm. might cause a stall and because we need more instructions
when replacing the mul we generate max. 3 instructions to replace this mul }
maxeffort:=3;
{ if the constant is not a shifter op, we need either some mov/mvn/bic/or sequence or
a ldr, so generating one more operation to replace this is beneficial }
if not(is_shifter_const(dword(a),b)) and not(is_shifter_const(not(dword(a)),b)) then
inc(maxeffort);
{ if the upper 5 bits are all set or clear, mul is one cycle faster }
if ((dword(a) and $f8000000)=0) or ((dword(a) and $f8000000)=$f8000000) then
dec(maxeffort);
{ if the upper 17 bits are all set or clear, mul is another cycle faster }
if ((dword(a) and $ffff8000)=0) or ((dword(a) and $ffff8000)=$ffff8000) then
dec(maxeffort);
{ most simple cases }
if a=1 then
a_load_reg_reg(list,OS_32,OS_32,src,dst)
@ -690,8 +714,8 @@ unit cgcpu;
however, the least significant bit is for free, it can be hidden in the initial
instruction
}
else if (bitsset+cycles<=3) and
(bitsset>popcnt(dword(nextpowerof2(multiplier,power)-multiplier) and $fffffffe)) then
else if (bitsset+cycles<=maxeffort) and
(bitsset<=popcnt(dword(nextpowerof2(multiplier,power)-multiplier) and $fffffffe)) then
begin
first:=true;
while multiplier<>0 do
@ -714,7 +738,7 @@ unit cgcpu;
list.concat(taicpu.op_reg_reg_const(A_RSB,dst,dst,0));
end
{ subtract from the next greater power of two? }
else if popcnt(dword(nextpowerof2(multiplier,power)-multiplier) and $fffffffe)+cycles<=3 then
else if popcnt(dword(nextpowerof2(multiplier,power)-multiplier) and $fffffffe)+cycles+1<=maxeffort then
begin
first:=true;
while multiplier<>0 do

25
tests/test/tmul1.pp
View File

@ -2,6 +2,31 @@ var
i : longint;
begin
i:=5;
i:=i*$80010;
if i<>2621520 then
halt(1);
i:=5;
i:=i*$18000010;
if i<>2013266000 then
halt(1);
i:=5;
i:=i*$18ffffef;
if i<>2097151915 then
halt(1);
i:=5;
i:=i*$7ffef;
if i<>2621355 then
halt(1);
i:=5;
i:=i*$6fffffcf;
if i<>805306123 then
halt(1);
i:=5;
i:=i*10;
i:=i*62;