llvm/lib/Target/X86/README-X86-64.txt - llvm-project - Git at Google

 //===- README_X86_64.txt - Notes for X86-64 code gen ----------------------===//

 AMD64 Optimization Manual 8.2 has some nice information about optimizing integer
 multiplication by a constant. How much of it applies to Intel's X86-64
 implementation? There are definite trade-offs to consider: latency vs. register
 pressure vs. code size.

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

 Are we better off using branches instead of cmove to implement FP to
 unsigned i64?

 _conv:
 	ucomiss	LC0(%rip), %xmm0
 	cvttss2siq	%xmm0, %rdx
 	jb	L3
 	subss	LC0(%rip), %xmm0
 	movabsq	$-9223372036854775808, %rax
 	cvttss2siq	%xmm0, %rdx
 	xorq	%rax, %rdx
 L3:
 	movq	%rdx, %rax
 	ret

 instead of

 _conv:
 	movss LCPI1_0(%rip), %xmm1
 	cvttss2siq %xmm0, %rcx
 	movaps %xmm0, %xmm2
 	subss %xmm1, %xmm2
 	cvttss2siq %xmm2, %rax
 	movabsq $-9223372036854775808, %rdx
 	xorq %rdx, %rax
 	ucomiss %xmm1, %xmm0
 	cmovb %rcx, %rax
 	ret

 Seems like the jb branch has high likelihood of being taken. It would have
 saved a few instructions.

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

 It's not possible to reference AH, BH, CH, and DH registers in an instruction
 requiring REX prefix. However, divb and mulb both produce results in AH. If isel
 emits a CopyFromReg which gets turned into a movb and that can be allocated a
 r8b - r15b.

 To get around this, isel emits a CopyFromReg from AX and then right shift it
 down by 8 and truncate it. It's not pretty but it works. We need some register
 allocation magic to make the hack go away (e.g. putting additional constraints
 on the result of the movb).

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

 The x86-64 ABI for hidden-argument struct returns requires that the
 incoming value of %rdi be copied into %rax by the callee upon return.

 The idea is that it saves callers from having to remember this value,
 which would often require a callee-saved register. Callees usually
 need to keep this value live for most of their body anyway, so it
 doesn't add a significant burden on them.

 We currently implement this in codegen, however this is suboptimal
 because it means that it would be quite awkward to implement the
 optimization for callers.

 A better implementation would be to relax the LLVM IR rules for sret
 arguments to allow a function with an sret argument to have a non-void
 return type, and to have the front-end to set up the sret argument value
 as the return value of the function. The front-end could more easily
 emit uses of the returned struct value to be in terms of the function's
 lowered return value, and it would free non-C frontends from a
 complication only required by a C-based ABI.

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

 We get a redundant zero extension for code like this:

 int mask[1000];
 int foo(unsigned x) {
  if (x < 10)
    x = x * 45;
  else
    x = x * 78;
  return mask[x];
 }

 _foo:
 LBB1_0:	## entry
 	cmpl	$9, %edi
 	jbe	LBB1_3	## bb
 LBB1_1:	## bb1
 	imull	$78, %edi, %eax
 LBB1_2:	## bb2
 	movl	%eax, %eax                    <----
 	movq	_mask@GOTPCREL(%rip), %rcx
 	movl	(%rcx,%rax,4), %eax
 	ret
 LBB1_3:	## bb
 	imull	$45, %edi, %eax
 	jmp	LBB1_2	## bb2

 Before regalloc, we have:

         %reg1025 = IMUL32rri8 %reg1024, 45, implicit-def %eflags
         JMP mbb<bb2,0x203afb0>
     Successors according to CFG: 0x203afb0 (#3)

 bb1: 0x203af60, LLVM BB @0x1e02310, ID#2:
     Predecessors according to CFG: 0x203aec0 (#0)
         %reg1026 = IMUL32rri8 %reg1024, 78, implicit-def %eflags
     Successors according to CFG: 0x203afb0 (#3)

 bb2: 0x203afb0, LLVM BB @0x1e02340, ID#3:
     Predecessors according to CFG: 0x203af10 (#1) 0x203af60 (#2)
         %reg1027 = PHI %reg1025, mbb<bb,0x203af10>,
                             %reg1026, mbb<bb1,0x203af60>
         %reg1029 = MOVZX64rr32 %reg1027

 so we'd have to know that IMUL32rri8 leaves the high word zero extended and to
 be able to recognize the zero extend.  This could also presumably be implemented
 if we have whole-function selectiondags.

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

 Take the following code
 (from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34653):
 extern unsigned long table[];
 unsigned long foo(unsigned char *p) {
   unsigned long tag = *p;
   return table[tag >> 4] + table[tag & 0xf];
 }

 Current code generated:
 	movzbl	(%rdi), %eax
 	movq	%rax, %rcx
 	andq	$240, %rcx
 	shrq	%rcx
 	andq	$15, %rax
 	movq	table(,%rax,8), %rax
 	addq	table(%rcx), %rax
 	ret

 Issues:
 1. First movq should be movl; saves a byte.
 2. Both andq's should be andl; saves another two bytes.  I think this was
    implemented at one point, but subsequently regressed.
 3. shrq should be shrl; saves another byte.
 4. The first andq can be completely eliminated by using a slightly more
    expensive addressing mode.

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

 Consider the following (contrived testcase, but contains common factors):

 #include <stdarg.h>
 int test(int x, ...) {
   int sum, i;
   va_list l;
   va_start(l, x);
   for (i = 0; i < x; i++)
     sum += va_arg(l, int);
   va_end(l);
   return sum;
 }

 Testcase given in C because fixing it will likely involve changing the IR
 generated for it.  The primary issue with the result is that it doesn't do any
 of the optimizations which are possible if we know the address of a va_list
 in the current function is never taken:
 1. We shouldn't spill the XMM registers because we only call va_arg with "int".
 2. It would be nice if we could sroa the va_list.
 3. Probably overkill, but it'd be cool if we could peel off the first five
 iterations of the loop.

 Other optimizations involving functions which use va_arg on floats which don't
 have the address of a va_list taken:
 1. Conversely to the above, we shouldn't spill general registers if we only
    call va_arg on "double".
 2. If we know nothing more than 64 bits wide is read from the XMM registers,
    we can change the spilling code to reduce the amount of stack used by half.

 //===---------------------------------------------------------------------===//
	//===- README_X86_64.txt - Notes for X86-64 code gen ----------------------===//

	AMD64 Optimization Manual 8.2 has some nice information about optimizing integer
	multiplication by a constant. How much of it applies to Intel's X86-64
	implementation? There are definite trade-offs to consider: latency vs. register
	pressure vs. code size.

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

	Are we better off using branches instead of cmove to implement FP to
	unsigned i64?

	_conv:
	ucomiss LC0(%rip), %xmm0
	cvttss2siq %xmm0, %rdx
	jb L3
	subss LC0(%rip), %xmm0
	movabsq $-9223372036854775808, %rax
	cvttss2siq %xmm0, %rdx
	xorq %rax, %rdx
	L3:
	movq %rdx, %rax
	ret

	instead of

	_conv:
	movss LCPI1_0(%rip), %xmm1
	cvttss2siq %xmm0, %rcx
	movaps %xmm0, %xmm2
	subss %xmm1, %xmm2
	cvttss2siq %xmm2, %rax
	movabsq $-9223372036854775808, %rdx
	xorq %rdx, %rax
	ucomiss %xmm1, %xmm0
	cmovb %rcx, %rax
	ret

	Seems like the jb branch has high likelihood of being taken. It would have
	saved a few instructions.

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

	It's not possible to reference AH, BH, CH, and DH registers in an instruction
	requiring REX prefix. However, divb and mulb both produce results in AH. If isel
	emits a CopyFromReg which gets turned into a movb and that can be allocated a
	r8b - r15b.

	To get around this, isel emits a CopyFromReg from AX and then right shift it
	down by 8 and truncate it. It's not pretty but it works. We need some register
	allocation magic to make the hack go away (e.g. putting additional constraints
	on the result of the movb).

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

	The x86-64 ABI for hidden-argument struct returns requires that the
	incoming value of %rdi be copied into %rax by the callee upon return.

	The idea is that it saves callers from having to remember this value,
	which would often require a callee-saved register. Callees usually
	need to keep this value live for most of their body anyway, so it
	doesn't add a significant burden on them.

	We currently implement this in codegen, however this is suboptimal
	because it means that it would be quite awkward to implement the
	optimization for callers.

	A better implementation would be to relax the LLVM IR rules for sret
	arguments to allow a function with an sret argument to have a non-void
	return type, and to have the front-end to set up the sret argument value
	as the return value of the function. The front-end could more easily
	emit uses of the returned struct value to be in terms of the function's
	lowered return value, and it would free non-C frontends from a
	complication only required by a C-based ABI.

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

	We get a redundant zero extension for code like this:

	int mask[1000];
	int foo(unsigned x) {
	if (x < 10)
	x = x * 45;
	else
	x = x * 78;
	return mask[x];
	}

	_foo:
	LBB1_0: ## entry
	cmpl $9, %edi
	jbe LBB1_3 ## bb
	LBB1_1: ## bb1
	imull $78, %edi, %eax
	LBB1_2: ## bb2
	movl %eax, %eax <----
	movq _mask@GOTPCREL(%rip), %rcx
	movl (%rcx,%rax,4), %eax
	ret
	LBB1_3: ## bb
	imull $45, %edi, %eax
	jmp LBB1_2 ## bb2

	Before regalloc, we have:

	%reg1025 = IMUL32rri8 %reg1024, 45, implicit-def %eflags
	JMP mbb<bb2,0x203afb0>
	Successors according to CFG: 0x203afb0 (#3)

	bb1: 0x203af60, LLVM BB @0x1e02310, ID#2:
	Predecessors according to CFG: 0x203aec0 (#0)
	%reg1026 = IMUL32rri8 %reg1024, 78, implicit-def %eflags
	Successors according to CFG: 0x203afb0 (#3)

	bb2: 0x203afb0, LLVM BB @0x1e02340, ID#3:
	Predecessors according to CFG: 0x203af10 (#1) 0x203af60 (#2)
	%reg1027 = PHI %reg1025, mbb<bb,0x203af10>,
	%reg1026, mbb<bb1,0x203af60>
	%reg1029 = MOVZX64rr32 %reg1027

	so we'd have to know that IMUL32rri8 leaves the high word zero extended and to
	be able to recognize the zero extend. This could also presumably be implemented
	if we have whole-function selectiondags.

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

	Take the following code
	(from http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34653):
	extern unsigned long table[];
	unsigned long foo(unsigned char *p) {
	unsigned long tag = *p;
	return table[tag >> 4] + table[tag & 0xf];
	}

	Current code generated:
	movzbl (%rdi), %eax
	movq %rax, %rcx
	andq $240, %rcx
	shrq %rcx
	andq $15, %rax
	movq table(,%rax,8), %rax
	addq table(%rcx), %rax
	ret

	Issues:
	1. First movq should be movl; saves a byte.
	2. Both andq's should be andl; saves another two bytes. I think this was
	implemented at one point, but subsequently regressed.
	3. shrq should be shrl; saves another byte.
	4. The first andq can be completely eliminated by using a slightly more
	expensive addressing mode.

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

	Consider the following (contrived testcase, but contains common factors):

	#include <stdarg.h>
	int test(int x, ...) {
	int sum, i;
	va_list l;
	va_start(l, x);
	for (i = 0; i < x; i++)
	sum += va_arg(l, int);
	va_end(l);
	return sum;
	}

	Testcase given in C because fixing it will likely involve changing the IR
	generated for it. The primary issue with the result is that it doesn't do any
	of the optimizations which are possible if we know the address of a va_list
	in the current function is never taken:
	1. We shouldn't spill the XMM registers because we only call va_arg with "int".
	2. It would be nice if we could sroa the va_list.
	3. Probably overkill, but it'd be cool if we could peel off the first five
	iterations of the loop.

	Other optimizations involving functions which use va_arg on floats which don't
	have the address of a va_list taken:
	1. Conversely to the above, we shouldn't spill general registers if we only
	call va_arg on "double".
	2. If we know nothing more than 64 bits wide is read from the XMM registers,
	we can change the spilling code to reduce the amount of stack used by half.

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