llvm/lib/CodeGen/README.txt - toolchain/llvm-project - Gitiles

 //===---------------------------------------------------------------------===//

 Common register allocation / spilling problem:

         mul lr, r4, lr
         str lr, [sp, #+52]
         ldr lr, [r1, #+32]
         sxth r3, r3
         ldr r4, [sp, #+52]
         mla r4, r3, lr, r4

 can be:

         mul lr, r4, lr
         mov r4, lr
         str lr, [sp, #+52]
         ldr lr, [r1, #+32]
         sxth r3, r3
         mla r4, r3, lr, r4

 and then "merge" mul and mov:

         mul r4, r4, lr
         str r4, [sp, #+52]
         ldr lr, [r1, #+32]
         sxth r3, r3
         mla r4, r3, lr, r4

 It also increase the likelihood the store may become dead.

 //===---------------------------------------------------------------------===//

 bb27 ...
         ...
         %reg1037 = ADDri %reg1039, 1
         %reg1038 = ADDrs %reg1032, %reg1039, %NOREG, 10
     Successors according to CFG: 0x8b03bf0 (#5)

 bb76 (0x8b03bf0, LLVM BB @0x8b032d0, ID#5):
     Predecessors according to CFG: 0x8b0c5f0 (#3) 0x8b0a7c0 (#4)
         %reg1039 = PHI %reg1070, mbb<bb76.outer,0x8b0c5f0>, %reg1037, mbb<bb27,0x8b0a7c0>

 Note ADDri is not a two-address instruction. However, its result %reg1037 is an
 operand of the PHI node in bb76 and its operand %reg1039 is the result of the
 PHI node. We should treat it as a two-address code and make sure the ADDri is
 scheduled after any node that reads %reg1039.

 //===---------------------------------------------------------------------===//

 Use local info (i.e. register scavenger) to assign it a free register to allow
 reuse:
         ldr r3, [sp, #+4]
         add r3, r3, #3
         ldr r2, [sp, #+8]
         add r2, r2, #2
         ldr r1, [sp, #+4]  <==
         add r1, r1, #1
         ldr r0, [sp, #+4]
         add r0, r0, #2

 //===---------------------------------------------------------------------===//

 LLVM aggressively lift CSE out of loop. Sometimes this can be negative side-
 effects:

 R1 = X + 4
 R2 = X + 7
 R3 = X + 15

 loop:
 load [i + R1]
 ...
 load [i + R2]
 ...
 load [i + R3]

 Suppose there is high register pressure, R1, R2, R3, can be spilled. We need
 to implement proper re-materialization to handle this:

 R1 = X + 4
 R2 = X + 7
 R3 = X + 15

 loop:
 R1 = X + 4  @ re-materialized
 load [i + R1]
 ...
 R2 = X + 7 @ re-materialized
 load [i + R2]
 ...
 R3 = X + 15 @ re-materialized
 load [i + R3]

 Furthermore, with re-association, we can enable sharing:

 R1 = X + 4
 R2 = X + 7
 R3 = X + 15

 loop:
 T = i + X
 load [T + 4]
 ...
 load [T + 7]
 ...
 load [T + 15]
 //===---------------------------------------------------------------------===//

 It's not always a good idea to choose rematerialization over spilling. If all
 the load / store instructions would be folded then spilling is cheaper because
 it won't require new live intervals / registers. See 2003-05-31-LongShifts for
 an example.

 //===---------------------------------------------------------------------===//

 With a copying garbage collector, derived pointers must not be retained across
 collector safe points; the collector could move the objects and invalidate the
 derived pointer. This is bad enough in the first place, but safe points can
 crop up unpredictably. Consider:

         %array = load { i32, [0 x %obj] }** %array_addr
         %nth_el = getelementptr { i32, [0 x %obj] }* %array, i32 0, i32 %n
         %old = load %obj** %nth_el
         %z = div i64 %x, %y
         store %obj* %new, %obj** %nth_el

 If the i64 division is lowered to a libcall, then a safe point will (must)
 appear for the call site. If a collection occurs, %array and %nth_el no longer
 point into the correct object.

 The fix for this is to copy address calculations so that dependent pointers
 are never live across safe point boundaries. But the loads cannot be copied
 like this if there was an intervening store, so may be hard to get right.

 Only a concurrent mutator can trigger a collection at the libcall safe point.
 So single-threaded programs do not have this requirement, even with a copying
 collector. Still, LLVM optimizations would probably undo a front-end's careful
 work.

 //===---------------------------------------------------------------------===//

 The ocaml frametable structure supports liveness information. It would be good
 to support it.

 //===---------------------------------------------------------------------===//

 The FIXME in ComputeCommonTailLength in BranchFolding.cpp needs to be
 revisited. The check is there to work around a misuse of directives in inline
 assembly.

 //===---------------------------------------------------------------------===//

 It would be good to detect collector/target compatibility instead of silently
 doing the wrong thing.

 //===---------------------------------------------------------------------===//

 It would be really nice to be able to write patterns in .td files for copies,
 which would eliminate a bunch of explicit predicates on them (e.g. no side
 effects).  Once this is in place, it would be even better to have tblgen
 synthesize the various copy insertion/inspection methods in TargetInstrInfo.

 //===---------------------------------------------------------------------===//

 Stack coloring improvements:

 1. Do proper LiveStackAnalysis on all stack objects including those which are
    not spill slots.
 2. Reorder objects to fill in gaps between objects.
    e.g. 4, 1, <gap>, 4, 1, 1, 1, <gap>, 4 => 4, 1, 1, 1, 1, 4, 4

 //===---------------------------------------------------------------------===//

 The scheduler should be able to sort nearby instructions by their address. For
 example, in an expanded memset sequence it's not uncommon to see code like this:

   movl $0, 4(%rdi)
   movl $0, 8(%rdi)
   movl $0, 12(%rdi)
   movl $0, 0(%rdi)

 Each of the stores is independent, and the scheduler is currently making an
 arbitrary decision about the order.

 //===---------------------------------------------------------------------===//

 Another opportunitiy in this code is that the $0 could be moved to a register:

   movl $0, 4(%rdi)
   movl $0, 8(%rdi)
   movl $0, 12(%rdi)
   movl $0, 0(%rdi)

 This would save substantial code size, especially for longer sequences like
 this. It would be easy to have a rule telling isel to avoid matching MOV32mi
 if the immediate has more than some fixed number of uses. It's more involved
 to teach the register allocator how to do late folding to recover from
 excessive register pressure.
	//===---------------------------------------------------------------------===//

	Common register allocation / spilling problem:

	mul lr, r4, lr
	str lr, [sp, #+52]
	ldr lr, [r1, #+32]
	sxth r3, r3
	ldr r4, [sp, #+52]
	mla r4, r3, lr, r4

	can be:

	mul lr, r4, lr
	mov r4, lr
	str lr, [sp, #+52]
	ldr lr, [r1, #+32]
	sxth r3, r3
	mla r4, r3, lr, r4

	and then "merge" mul and mov:

	mul r4, r4, lr
	str r4, [sp, #+52]
	ldr lr, [r1, #+32]
	sxth r3, r3
	mla r4, r3, lr, r4

	It also increase the likelihood the store may become dead.

	//===---------------------------------------------------------------------===//

	bb27 ...
	...
	%reg1037 = ADDri %reg1039, 1
	%reg1038 = ADDrs %reg1032, %reg1039, %NOREG, 10
	Successors according to CFG: 0x8b03bf0 (#5)

	bb76 (0x8b03bf0, LLVM BB @0x8b032d0, ID#5):
	Predecessors according to CFG: 0x8b0c5f0 (#3) 0x8b0a7c0 (#4)
	%reg1039 = PHI %reg1070, mbb<bb76.outer,0x8b0c5f0>, %reg1037, mbb<bb27,0x8b0a7c0>

	Note ADDri is not a two-address instruction. However, its result %reg1037 is an
	operand of the PHI node in bb76 and its operand %reg1039 is the result of the
	PHI node. We should treat it as a two-address code and make sure the ADDri is
	scheduled after any node that reads %reg1039.

	//===---------------------------------------------------------------------===//

	Use local info (i.e. register scavenger) to assign it a free register to allow
	reuse:
	ldr r3, [sp, #+4]
	add r3, r3, #3
	ldr r2, [sp, #+8]
	add r2, r2, #2
	ldr r1, [sp, #+4] <==
	add r1, r1, #1
	ldr r0, [sp, #+4]
	add r0, r0, #2

	//===---------------------------------------------------------------------===//

	LLVM aggressively lift CSE out of loop. Sometimes this can be negative side-
	effects:

	R1 = X + 4
	R2 = X + 7
	R3 = X + 15

	loop:
	load [i + R1]
	...
	load [i + R2]
	...
	load [i + R3]

	Suppose there is high register pressure, R1, R2, R3, can be spilled. We need
	to implement proper re-materialization to handle this:

	R1 = X + 4
	R2 = X + 7
	R3 = X + 15

	loop:
	R1 = X + 4 @ re-materialized
	load [i + R1]
	...
	R2 = X + 7 @ re-materialized
	load [i + R2]
	...
	R3 = X + 15 @ re-materialized
	load [i + R3]

	Furthermore, with re-association, we can enable sharing:

	R1 = X + 4
	R2 = X + 7
	R3 = X + 15

	loop:
	T = i + X
	load [T + 4]
	...
	load [T + 7]
	...
	load [T + 15]
	//===---------------------------------------------------------------------===//

	It's not always a good idea to choose rematerialization over spilling. If all
	the load / store instructions would be folded then spilling is cheaper because
	it won't require new live intervals / registers. See 2003-05-31-LongShifts for
	an example.

	//===---------------------------------------------------------------------===//

	With a copying garbage collector, derived pointers must not be retained across
	collector safe points; the collector could move the objects and invalidate the
	derived pointer. This is bad enough in the first place, but safe points can
	crop up unpredictably. Consider:

	%array = load { i32, [0 x %obj] }** %array_addr
	%nth_el = getelementptr { i32, [0 x %obj] }* %array, i32 0, i32 %n
	%old = load %obj** %nth_el
	%z = div i64 %x, %y
	store %obj* %new, %obj** %nth_el

	If the i64 division is lowered to a libcall, then a safe point will (must)
	appear for the call site. If a collection occurs, %array and %nth_el no longer
	point into the correct object.

	The fix for this is to copy address calculations so that dependent pointers
	are never live across safe point boundaries. But the loads cannot be copied
	like this if there was an intervening store, so may be hard to get right.

	Only a concurrent mutator can trigger a collection at the libcall safe point.
	So single-threaded programs do not have this requirement, even with a copying
	collector. Still, LLVM optimizations would probably undo a front-end's careful
	work.

	//===---------------------------------------------------------------------===//

	The ocaml frametable structure supports liveness information. It would be good
	to support it.

	//===---------------------------------------------------------------------===//

	The FIXME in ComputeCommonTailLength in BranchFolding.cpp needs to be
	revisited. The check is there to work around a misuse of directives in inline
	assembly.

	//===---------------------------------------------------------------------===//

	It would be good to detect collector/target compatibility instead of silently
	doing the wrong thing.

	//===---------------------------------------------------------------------===//

	It would be really nice to be able to write patterns in .td files for copies,
	which would eliminate a bunch of explicit predicates on them (e.g. no side
	effects). Once this is in place, it would be even better to have tblgen
	synthesize the various copy insertion/inspection methods in TargetInstrInfo.

	//===---------------------------------------------------------------------===//

	Stack coloring improvements:

	1. Do proper LiveStackAnalysis on all stack objects including those which are
	not spill slots.
	2. Reorder objects to fill in gaps between objects.
	e.g. 4, 1, <gap>, 4, 1, 1, 1, <gap>, 4 => 4, 1, 1, 1, 1, 4, 4

	//===---------------------------------------------------------------------===//

	The scheduler should be able to sort nearby instructions by their address. For
	example, in an expanded memset sequence it's not uncommon to see code like this:

	movl $0, 4(%rdi)
	movl $0, 8(%rdi)
	movl $0, 12(%rdi)
	movl $0, 0(%rdi)

	Each of the stores is independent, and the scheduler is currently making an
	arbitrary decision about the order.

	//===---------------------------------------------------------------------===//

	Another opportunitiy in this code is that the $0 could be moved to a register:

	movl $0, 4(%rdi)
	movl $0, 8(%rdi)
	movl $0, 12(%rdi)
	movl $0, 0(%rdi)

	This would save substantial code size, especially for longer sequences like
	this. It would be easy to have a rule telling isel to avoid matching MOV32mi
	if the immediate has more than some fixed number of uses. It's more involved
	to teach the register allocator how to do late folding to recover from
	excessive register pressure.