1) For 8-bit registers try to use first the ones that are parts of the
same register (AL then AH). This way we only alias 2 16/32-bit
registers after allocating 4 8-bit variables.
2) Move EBX as the last register to allocate. This will cause less
spills to happen since we will have 8-bit registers available up to
register excaustion (assuming we use the allocation order). It
would be nice if we could push all of the 8-bit aliased registers
towards the end but we much prefer to keep callee saved register to
the end to avoid saving them on entry and exit of the function.
For example this gives a slight reduction of spills with linear scan
on 164.gzip.
Before:
11221 asm-printer - Number of machine instrs printed
975 spiller - Number of loads added
675 spiller - Number of stores added
398 spiller - Number of register spills
After:
11182 asm-printer - Number of machine instrs printed
952 spiller - Number of loads added
652 spiller - Number of stores added
386 spiller - Number of register spills
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their names more decriptive. A name consists of the base name, a
default operand size followed by a character per operand with an
optional special size. For example:
ADD8rr -> add, 8-bit register, 8-bit register
IMUL16rmi -> imul, 16-bit register, 16-bit memory, 16-bit immediate
IMUL16rmi8 -> imul, 16-bit register, 16-bit memory, 8-bit immediate
MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
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parse. The name is now I (operand size)*. For example:
Im32 -> instruction with 32-bit memory operands.
Im16i8 -> instruction with 16-bit memory operands and 8 bit immediate
operands.
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the size of the immediate and the memory operand on instructions that
use them. This resolves problems with instructions that take both a
memory and an immediate operand but their sizes differ (i.e. ADDmi32b).
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an 8-bit immediate. So mark the shifts that take immediates as taking
an 8-bit argument. The rest with the implicit use of CL are marked
appropriately.
A bug still exists:
def SHLDmri32 : I2A8 <"shld", 0xA4, MRMDestMem>, TB; // [mem32] <<= [mem32],R32 imm8
The immediate in the above instruction is 8-bit but the memory
reference is 32-bit. The printer prints this as an 8-bit reference
which confuses the assembler. Same with SHRDmri32.
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scaled indexes. This allows us to compile GEP's like this:
int* %test([10 x { int, { int } }]* %X, int %Idx) {
%Idx = cast int %Idx to long
%X = getelementptr [10 x { int, { int } }]* %X, long 0, long %Idx, ubyte 1, ubyte 0
ret int* %X
}
Into a single address computation:
test:
mov %EAX, DWORD PTR [%ESP + 4]
mov %ECX, DWORD PTR [%ESP + 8]
lea %EAX, DWORD PTR [%EAX + 8*%ECX + 4]
ret
Before it generated:
test:
mov %EAX, DWORD PTR [%ESP + 4]
mov %ECX, DWORD PTR [%ESP + 8]
shl %ECX, 3
add %EAX, %ECX
lea %EAX, DWORD PTR [%EAX + 4]
ret
This is useful for things like int/float/double arrays, as the indexing can be folded into
the loads&stores, reducing register pressure and decreasing the pressure on the decode unit.
With these changes, I expect our performance on 256.bzip2 and gzip to improve a lot. On
bzip2 for example, we go from this:
10665 asm-printer - Number of machine instrs printed
40 ra-local - Number of loads/stores folded into instructions
1708 ra-local - Number of loads added
1532 ra-local - Number of stores added
1354 twoaddressinstruction - Number of instructions added
1354 twoaddressinstruction - Number of two-address instructions
2794 x86-peephole - Number of peephole optimization performed
to this:
9873 asm-printer - Number of machine instrs printed
41 ra-local - Number of loads/stores folded into instructions
1710 ra-local - Number of loads added
1521 ra-local - Number of stores added
789 twoaddressinstruction - Number of instructions added
789 twoaddressinstruction - Number of two-address instructions
2142 x86-peephole - Number of peephole optimization performed
... and these types of instructions are often in tight loops.
Linear scan is also helped, but not as much. It goes from:
8787 asm-printer - Number of machine instrs printed
2389 liveintervals - Number of identity moves eliminated after coalescing
2288 liveintervals - Number of interval joins performed
3522 liveintervals - Number of intervals after coalescing
5810 liveintervals - Number of original intervals
700 spiller - Number of loads added
487 spiller - Number of stores added
303 spiller - Number of register spills
1354 twoaddressinstruction - Number of instructions added
1354 twoaddressinstruction - Number of two-address instructions
363 x86-peephole - Number of peephole optimization performed
to:
7982 asm-printer - Number of machine instrs printed
1759 liveintervals - Number of identity moves eliminated after coalescing
1658 liveintervals - Number of interval joins performed
3282 liveintervals - Number of intervals after coalescing
4940 liveintervals - Number of original intervals
635 spiller - Number of loads added
452 spiller - Number of stores added
288 spiller - Number of register spills
789 twoaddressinstruction - Number of instructions added
789 twoaddressinstruction - Number of two-address instructions
258 x86-peephole - Number of peephole optimization performed
Though I'm not complaining about the drop in the number of intervals. :)
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to do analysis.
*** FOLD getelementptr instructions into loads and stores when possible,
making use of some of the crazy X86 addressing modes.
For example, the following C++ program fragment:
struct complex {
double re, im;
complex(double r, double i) : re(r), im(i) {}
};
inline complex operator+(const complex& a, const complex& b) {
return complex(a.re+b.re, a.im+b.im);
}
complex addone(const complex& arg) {
return arg + complex(1,0);
}
Used to be compiled to:
_Z6addoneRK7complex:
mov %EAX, DWORD PTR [%ESP + 4]
mov %ECX, DWORD PTR [%ESP + 8]
*** mov %EDX, %ECX
fld QWORD PTR [%EDX]
fld1
faddp %ST(1)
*** add %ECX, 8
fld QWORD PTR [%ECX]
fldz
faddp %ST(1)
*** mov %ECX, %EAX
fxch %ST(1)
fstp QWORD PTR [%ECX]
*** add %EAX, 8
fstp QWORD PTR [%EAX]
ret
Now it is compiled to:
_Z6addoneRK7complex:
mov %EAX, DWORD PTR [%ESP + 4]
mov %ECX, DWORD PTR [%ESP + 8]
fld QWORD PTR [%ECX]
fld1
faddp %ST(1)
fld QWORD PTR [%ECX + 8]
fldz
faddp %ST(1)
fxch %ST(1)
fstp QWORD PTR [%EAX]
fstp QWORD PTR [%EAX + 8]
ret
Other programs should see similar improvements, across the board. Note that
in addition to reducing instruction count, this also reduces register pressure
a lot, always a good thing on X86. :)
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into a single LEA instruction. This should improve the code generated for
things like X->A.B.C[12].D.
The bigger benefit is still coming though. Note that this uses an LEA instruction
instead of an add, giving the register allocator more freedom. We should probably
never generate ADDri32's.
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block into MachineBasicBlock::getFirstTerminator().
This also fixes a bug in the implementation of the above in both
RegAllocLocal and InstrSched, where instructions where added after the
terminator if the basic block's only instruction was a terminator (it
shouldn't matter for RegAllocLocal since this case never occurs in
practice).
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use FP instructions. This reduces the number of instructions inserted in
176.gcc (for example) from 58074 to 101 (it doesn't use much FP, which
is typical). This reduction speeds up the entire code generator. In the
case of 176.gcc, llc went from taking 31.38s to 24.78s. The passes that
sped up the most are the register allocator and the 2 live variable analysis
passes, which sped up 2.3, 1.3, and 1.5s respectively. The asmprinter
pass also sped up because it doesn't print the instructions in comments :)
Note that this patch is likely to expose latent bugs in machine code passes,
because now basicblock can be empty, where they were never empty before. I
cleaned out regalloclocal, but who knows about linscan :)
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switch statements in the constructors and simplifies the
implementation of the getUseType() member function. You will have to
specify defs using MachineOperand::Def instead of MOTy::Def though
(similarly for Use and UseAndDef).
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(minor) benefits right now:
1. An extra dummy MOVrr32 is gone. This move would often be coallesced by
both allocators anyway.
2. The code now uses the gep_type_iterator to walk the gep, which should future
proof it a bit. It still assumes that array indexes are Longs though.
These don't really justify rewriting the code. The big benefit will come later
though.
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value is a physreg and one is a virtreg. For this reason, disable copy folding
entirely for physregs. Also, use the new isMoveInstr target hook which gives us
folding of FP moves as well.
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MRegisterInfo::getNumRegs() instead of
MRegisterInfo::FirstVirtualRegister.
Also use MRegisterInfo::is{Physical,Virtual}Register where
appropriate.
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that will be responsible for the creation of MachineFunctions and will
be required by all MachineFunctionPass passes.
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ilist of MachineInstr objects. This allows constant time removal and
insertion of MachineInstr instances from anywhere in each
MachineBasicBlock. It also allows for constant time splicing of
MachineInstrs into or out of MachineBasicBlocks.
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