On 8/23/23 14:13, Jeff Law wrote:
This is primarily Jivan's work, I'm mostly responsible for the write-up
and coordinating with Vlad on a few questions.
On targets with limitations on immediates usable in arithmetic
instructions, LRA's register elimination phase can construct fairly poor
code.
This example (from the GCC testsuite) illustrates the problem well.
int consume (void *);
int foo (void) {
int x[1000000];
return consume (x + 1000);
}
If you compile on riscv64-linux-gnu with "-O2 -march=rv64gc
-mabi=lp64d", then you'll get this code (up to the call to consume()).
.cfi_startproc
li t0,-4001792
li a0,-3997696
li a5,4001792
addi sp,sp,-16
.cfi_def_cfa_offset 16
addi t0,t0,1792
addi a0,a0,1696
addi a5,a5,-1792
sd ra,8(sp)
add a5,a5,a0
add sp,sp,t0
.cfi_def_cfa_offset 4000016
.cfi_offset 1, -8
add a0,a5,sp
call consume
Of particular interest is the value in a0 when we call consume. We
compute that horribly inefficiently. If we back-substitute from the
final assignment to a0 we get...
a0 = a5 + sp
a0 = a5 + (sp + t0)
a0 = (a5 + a0) + (sp + t0)
a0 = ((a5 - 1792) + a0) + (sp + t0)
a0 = ((a5 - 1792) + (a0 + 1696)) + (sp + t0)
a0 = ((a5 - 1792) + (a0 + 1696)) + (sp + (t0 + 1792))
a0 = (a5 + (a0 + 1696)) + (sp + t0) // removed offsetting terms
a0 = (a5 + (a0 + 1696)) + ((sp - 16) + t0)
a0 = (4001792 + (a0 + 1696)) + ((sp - 16) + t0)
a0 = (4001792 + (-3997696 + 1696)) + ((sp - 16) + t0)
a0 = (4001792 + (-3997696 + 1696)) + ((sp - 16) + -4001792)
a0 = (-3997696 + 1696) + (sp -16) // removed offsetting terms
a0 = sp - 3990616
That's a pretty convoluted way to compute sp - 3990616.
Something like this would be notably better (not great, but we need both
the stack adjustment and the address of the object to pass to consume):
addi sp,sp,-16
sd ra,8(sp)
li t0,-4001792
addi t0,t0,1792
add sp,sp,t0
li a0,4096
addi a0,a0,-96
add a0,sp,a0
call consume
The problem is LRA's elimination code is not handling the case where we
have (plus (reg1) (reg2) where reg1 is an eliminable register and reg2
has a known equivalency, particularly a constant.
If we can determine that reg2 is equivalent to a constant and treat
(plus (reg1) (reg2)) in the same way we'd treat (plus (reg1)
(const_int)) then we can get the desired code.
This eliminates about 19b instructions, or roughly 1% for deepsjeng on
rv64. There are improvements elsewhere, but they're relatively small.
This may ultimately lessen the value of Manolis's fold-mem-offsets
patch. So we'll have to evaluate that again once he posts a new version.
Bootstrapped and regression tested on x86_64 as well as bootstrapped on
rv64. Earlier versions have been tested against spec2017. Pre-approved
by Vlad in a private email conversation (thanks Vlad!).
Committed to the trunk,
Whoops. Attached the wrong patch :-) This is the right one.
jeff
commit 6619b3d4c15cd754798b1048c67f3806bbcc2e6d
Author: Jivan Hakobyan <jivanhakoby...@gmail.com>
Date: Wed Aug 23 14:10:30 2023 -0600
Improve quality of code from LRA register elimination
This is primarily Jivan's work, I'm mostly responsible for the write-up and
coordinating with Vlad on a few questions.
On targets with limitations on immediates usable in arithmetic instructions,
LRA's register elimination phase can construct fairly poor code.
This example (from the GCC testsuite) illustrates the problem well.
int consume (void *);
int foo (void) {
int x[1000000];
return consume (x + 1000);
}
If you compile on riscv64-linux-gnu with "-O2 -march=rv64gc -mabi=lp64d",
then
you'll get this code (up to the call to consume()).
.cfi_startproc
li t0,-4001792
li a0,-3997696
li a5,4001792
addi sp,sp,-16
.cfi_def_cfa_offset 16
addi t0,t0,1792
addi a0,a0,1696
addi a5,a5,-1792
sd ra,8(sp)
add a5,a5,a0
add sp,sp,t0
.cfi_def_cfa_offset 4000016
.cfi_offset 1, -8
add a0,a5,sp
call consume
Of particular interest is the value in a0 when we call consume. We compute
that
horribly inefficiently. If we back-substitute from the final assignment
to a0
we get...
a0 = a5 + sp
a0 = a5 + (sp + t0)
a0 = (a5 + a0) + (sp + t0)
a0 = ((a5 - 1792) + a0) + (sp + t0)
a0 = ((a5 - 1792) + (a0 + 1696)) + (sp + t0)
a0 = ((a5 - 1792) + (a0 + 1696)) + (sp + (t0 + 1792))
a0 = (a5 + (a0 + 1696)) + (sp + t0) // removed offsetting terms
a0 = (a5 + (a0 + 1696)) + ((sp - 16) + t0)
a0 = (4001792 + (a0 + 1696)) + ((sp - 16) + t0)
a0 = (4001792 + (-3997696 + 1696)) + ((sp - 16) + t0)
a0 = (4001792 + (-3997696 + 1696)) + ((sp - 16) + -4001792)
a0 = (-3997696 + 1696) + (sp -16) // removed offsetting terms
a0 = sp - 3990616
That's a pretty convoluted way to compute sp - 3990616.
Something like this would be notably better (not great, but we need both the
stack adjustment and the address of the object to pass to consume):
addi sp,sp,-16
sd ra,8(sp)
li t0,-4001792
addi t0,t0,1792
add sp,sp,t0
li a0,4096
addi a0,a0,-96
add a0,sp,a0
call consume
The problem is LRA's elimination code is not handling the case where we have
(plus (reg1) (reg2) where reg1 is an eliminable register and reg2 has a
known
equivalency, particularly a constant.
If we can determine that reg2 is equivalent to a constant and treat (plus
(reg1) (reg2)) in the same way we'd treat (plus (reg1) (const_int)) then we
can
get the desired code.
This eliminates about 19b instructions, or roughly 1% for deepsjeng on rv64.
There are improvements elsewhere, but they're relatively small. This may
ultimately lessen the value of Manolis's fold-mem-offsets patch. So we'll
have
to evaluate that again once he posts a new version.
Bootstrapped and regression tested on x86_64 as well as bootstrapped on
rv64.
Earlier versions have been tested against spec2017. Pre-approved by Vlad
in a
private email conversation (thanks Vlad!).
Committed to the trunk,
gcc/
* lra-eliminations.cc (eliminate_regs_in_insn): Use equivalences to
to help simplify code further.
diff --git a/gcc/lra-eliminations.cc b/gcc/lra-eliminations.cc
index 3c58d4a3815..df613cdda76 100644
--- a/gcc/lra-eliminations.cc
+++ b/gcc/lra-eliminations.cc
@@ -926,6 +926,18 @@ eliminate_regs_in_insn (rtx_insn *insn, bool replace_p,
bool first_p,
/* First see if the source is of the form (plus (...) CST). */
if (plus_src && poly_int_rtx_p (XEXP (plus_src, 1), &offset))
plus_cst_src = plus_src;
+ /* If we are doing initial offset computation, then utilize
+ eqivalences to discover a constant for the second term
+ of PLUS_SRC. */
+ else if (plus_src && REG_P (XEXP (plus_src, 1)))
+ {
+ int regno = REGNO (XEXP (plus_src, 1));
+ if (regno < ira_reg_equiv_len
+ && ira_reg_equiv[regno].constant != NULL_RTX
+ && !replace_p
+ && poly_int_rtx_p (ira_reg_equiv[regno].constant, &offset))
+ plus_cst_src = plus_src;
+ }
/* Check that the first operand of the PLUS is a hard reg or
the lowpart subreg of one. */
if (plus_cst_src)
