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.
This directory contains the GNU Compiler Collection (GCC).
The GNU Compiler Collection is free software. See the files whose
names start with COPYING for copying permission. The manuals, and
some of the runtime libraries, are under different terms; see the
individual source files for details.
The directory INSTALL contains copies of the installation information
as HTML and plain text. The source of this information is
gcc/doc/install.texi. The installation information includes details
of what is included in the GCC sources and what files GCC installs.
See the file gcc/doc/gcc.texi (together with other files that it
includes) for usage and porting information. An online readable
version of the manual is in the files gcc/doc/gcc.info*.
See http://gcc.gnu.org/bugs/ for how to report bugs usefully.
Copyright years on GCC source files may be listed using range
notation, e.g., 1987-2012, indicating that every year in the range,
inclusive, is a copyrightable year that could otherwise be listed
individually.
6619b3d4c1