For example, MachineLICM should not hoist a load that is not guaranteed to be executed.
Radar 10254254.
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across unwind edges. This is for the back-end which expects such things.
The code is from the original SjLj EH pass.
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to the landing pad. This will be used by the back-end to generate the jump
tables for dispatching the arriving longjmp in sjlj eh.
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PhysReg operands are not allowed to have sub-register indices at all.
For virtual registers with sub-reg indices, check that all registers in
the register class support the sub-reg index.
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EXTRACT_SUBREG is emitted as %dst = COPY %src:sub, so there is no need to
constrain the %dst register class. RegisterCoalescer will apply the
necessary constraints if it decides to eliminate the COPY.
The %src register class does need to be constrained to something with
the right sub-registers, though. This is currently done manually with
COPY_TO_REGCLASS nodes. They can possibly be removed after this patch.
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The register class created by INSERT_SUBREG and SUBREG_TO_REG must be
legal and support the SubIdx sub-registers.
The new getSubClassWithSubReg() hook can compute that.
This may create INSERT_SUBREG instructions defining a larger register
class than the sub-register being inserted. That is OK,
RegisterCoalescer will constrain the register class as needed when it
eliminates the INSERT_SUBREG instructions.
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TwoAddressInstructionPass should annotate instructions with <undef>
flags when it lower REG_SEQUENCE instructions. LiveIntervals should not
be in the business of modifying code (except for kill flags, perhaps).
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For example:
%vreg10:dsub_0<def,undef> = COPY %vreg1
%vreg10:dsub_1<def> = COPY %vreg2
is rewritten as:
%D2<def> = COPY %D0, %Q1<imp-def>
%D3<def> = COPY %D1, %Q1<imp-use,kill>, %Q1<imp-def>
The first COPY doesn't care about the previous value of %Q1, so it
doesn't read that register.
The second COPY is a partial redefinition of %Q1, so it implicitly kills
and redefines that register.
This makes it possible to recognize instructions that can harmlessly
clobber the full super-register. The write and don't read the
super-register.
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RegisterCoalescer can create sub-register defs when it is joining a
register with a sub-register. Add <undef> flags to these new
sub-register defs where appropriate.
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The <undef> flag says that a MachineOperand doesn't read its register,
or doesn't depend on the previous value of its register.
A full register def never depends on the previous register value. A
partial register def may depend on the previous value if it is intended
to update part of a register.
For example:
%vreg10:dsub_0<def,undef> = COPY %vreg1
%vreg10:dsub_1<def> = COPY %vreg2
The first copy instruction defines the full %vreg10 register with the
bits not covered by dsub_0 defined as <undef>. It is not considered a
read of %vreg10.
The second copy modifies part of %vreg10 while preserving the rest. It
has an implicit read of %vreg10.
This patch adds a MachineOperand::readsReg() method to determine if an
operand reads its register.
Previously, this was modelled by adding a full-register <imp-def>
operand to the instruction. This approach makes it possible to
determine directly from a MachineOperand if it reads its register. No
scanning of MI operands is required.
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and the alignment is 0 (i.e., it's defined globally in one file and declared in
another file) it could get an alignment which is larger than the ABI allows for
that type, resulting in aligned moves being used for unaligned loads.
For instance, in file A.c:
struct S s;
In file B.c:
struct {
// something long
};
extern S s;
void foo() {
struct S p = s;
// ...
}
this copy is a 'memcpy' which is turned into a series of 'movaps' instructions
on X86. But this is wrong, because 'struct S' has alignment of 4, not 16.
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This helps with porting code from 2.9 to 3.0 as TargetSelect.h changed location,
and if you include the old one by accident you will trigger this assert.
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The function needs to scan the implicit operands anyway, so no
performance is won by caching the number of implicit operands added to
an instruction.
This also fixes a bug when adding operands after an implicit operand has
been added manually. The NumImplicitOps count wasn't kept up to date.
MachineInstr::addOperand() will now consistently place all explicit
operands before all the implicit operands, regardless of the order they
are added. It is possible to change an MI opcode and add additional
explicit operands. They will be inserted before any existing implicit
operands.
The only exception is inline asm instructions where operands are never
reordered. This is because of a hack that marks explicit clobber regs
on inline asm as <implicit-def> to please the fast register allocator.
This hack can go away when InstrEmitter and FastIsel can add exact
<dead> flags to physreg defs.
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Upon further review, most of the EH code should remain written at the IR
level. The part which breaks SSA form is the dispatch table, so that part will
be moved to the back-end.
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This intrinsic is used to pass the index of the function context to the back-end
for further processing. The back-end is in charge of filling in the rest of the
entries.
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The DWARF exception pass uses the call site information, which is set up here. A
pre-RA pass is too late for it to use this information. So create and setup the
function context here, and then insert the call site values here (and map the
call sites for the DWARF EH pass). This is simpler than the original pass, and
doesn't make the CFG lose its SSA-ness.
It's a win-win-win-win-lose-win-win situation.
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current IR-level pass.
The old SjLj EH pass has some problems, especially with the new EH model. Most
significantly, it violates some of the new restrictions the new model has. For
instance, the 'dispatch' table wants to jump to the landing pad, but we cannot
allow that because only an invoke's unwind edge can jump to a landing pad. This
requires us to mangle the code something awful. In addition, we need to keep the
now dead landingpad instructions around instead of CSE'ing them because the
DWARF emitter uses that information (they are dead because no control flow edge
will execute them - the control flow edge from an invoke's unwind is superceded
by the edge coming from the dispatch).
Basically, this pass belongs not at the IR level where SSA is king, but at the
code-gen level, where we have more flexibility.
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