On Thu, May 20, 2021 at 12:51 AM Richard Biener
<richard.guent...@gmail.com> wrote:
>
> On Wed, May 19, 2021 at 3:22 PM H.J. Lu <hjl.to...@gmail.com> wrote:
> >
> > On Wed, May 19, 2021 at 2:33 AM Richard Biener
> > <richard.guent...@gmail.com> wrote:
> > >
> > > On Tue, May 18, 2021 at 9:16 PM H.J. Lu <hjl.to...@gmail.com> wrote:
> > > >
> > > > When expanding a constant constructor, don't call expand_constructor if
> > > > it is more efficient to load the data from the memory via move by
> > > > pieces.
> > > >
> > > > gcc/
> > > >
> > > > PR middle-end/90773
> > > > * expr.c (expand_expr_real_1): Don't call expand_constructor if
> > > > it is more efficient to load the data from the memory.
> > > >
> > > > gcc/testsuite/
> > > >
> > > > PR middle-end/90773
> > > > * gcc.target/i386/pr90773-24.c: New test.
> > > > * gcc.target/i386/pr90773-25.c: Likewise.
> > > > ---
> > > > gcc/expr.c | 10 ++++++++++
> > > > gcc/testsuite/gcc.target/i386/pr90773-24.c | 22 ++++++++++++++++++++++
> > > > gcc/testsuite/gcc.target/i386/pr90773-25.c | 20 ++++++++++++++++++++
> > > > 3 files changed, 52 insertions(+)
> > > > create mode 100644 gcc/testsuite/gcc.target/i386/pr90773-24.c
> > > > create mode 100644 gcc/testsuite/gcc.target/i386/pr90773-25.c
> > > >
> > > > diff --git a/gcc/expr.c b/gcc/expr.c
> > > > index d09ee42e262..80e01ea1cbe 100644
> > > > --- a/gcc/expr.c
> > > > +++ b/gcc/expr.c
> > > > @@ -10886,6 +10886,16 @@ expand_expr_real_1 (tree exp, rtx target,
> > > > machine_mode tmode,
> > > > unsigned HOST_WIDE_INT ix;
> > > > tree field, value;
> > > >
> > > > + /* Check if it is more efficient to load the data from
> > > > + the memory directly. FIXME: How many stores do we
> > > > + need here if not moved by pieces? */
> > > > + unsigned HOST_WIDE_INT bytes
> > > > + = tree_to_uhwi (TYPE_SIZE_UNIT (type));
> > >
> > > that's prone to fail - it could be a VLA.
> >
> > What do you mean by fail? Is it ICE or missed optimization?
> > Do you have a testcase?
> >
> > >
> > > > + if ((bytes / UNITS_PER_WORD) > 2
> > > > + && MOVE_MAX_PIECES > UNITS_PER_WORD
> > > > + && can_move_by_pieces (bytes, TYPE_ALIGN (type)))
> > > > + goto normal_inner_ref;
> > > > +
> > >
> > > It looks like you're concerned about aggregate copies but this also
> > > handles
> > > non-aggregates (which on GIMPLE might already be optimized of course).
> >
> > Here I check if we copy more than 2 words and we can move more than
> > a word in a single instruction.
> >
> > > Also you say "if it's cheaper" but I see no cost considerations. How do
> > > we generally handle immed const vs. load from constant pool costs?
> >
> > This trades 2 (update to 8) stores with one load plus one store. Is there
> > a way to check which one is faster?
>
> I'm not sure - it depends on whether the target can do stores from immediates
> at all or what restrictions apply, what the immediate value actually is
> (zero or all-ones should be way cheaper than sth arbitrary) and how the
> pressure on the load unit is. can_move_by_pieces (bytes, TYPE_ALIGN (type))
> also does not guarantee it will actually move pieces larger than
> UNITS_PER_WORD,
> that might depend on alignment. There's by_pieces_ninsns that might provide
> some hint here.
>
> I'm sure it works well for x86.
>
> I wonder if the existing code is in the appropriate place and we
> shouldn't instead
> handle this somewhere upthread where we ask to copy 'exp' into some other
> memory location. For your testcase that's expand_assignment but I can
> imagine passing array[0] by value to a function resulting in similar copying.
> Testing that shows we get
>
> pushq array+56(%rip)
> .cfi_def_cfa_offset 24
> pushq array+48(%rip)
> .cfi_def_cfa_offset 32
> pushq array+40(%rip)
> .cfi_def_cfa_offset 40
> pushq array+32(%rip)
> .cfi_def_cfa_offset 48
> pushq array+24(%rip)
> .cfi_def_cfa_offset 56
> pushq array+16(%rip)
> .cfi_def_cfa_offset 64
> pushq array+8(%rip)
> .cfi_def_cfa_offset 72
> pushq array(%rip)
> .cfi_def_cfa_offset 80
> call bar
>
> for that. We do have the by-pieces infrastructure to generally do this kind
> of
> copying but in both of these cases we do not seem to use it. I also wonder
> if the by-pieces infrastructure can pick up constant initializers
> automagically
> (we could native_encode the initializer part and feed the by-pieces
> infrastructure with an array of bytes). There for example might be easy to
> immediate-store byte parts and difficult ones where we could decide on a
> case-by-case basis whether to load+store or immediate-store them.
I opened:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100704
> For example if I change your testcase to have the array[] initializer
> all-zero we currently emit
>
> pxor %xmm0, %xmm0
> movups %xmm0, (%rdi)
> movups %xmm0, 16(%rdi)
> movups %xmm0, 32(%rdi)
> movups %xmm0, 48(%rdi)
> ret
>
> will your patch cause us to emit 4 loads? OTHO if I do
>
> const struct S array[] = {
> { 0, 0, 0, 7241, 124764, 48, 16, 33, 10, 96, 2, 0, 0, 4 }
> };
>
> we get
>
> movq $0, (%rdi)
> movl $0, 8(%rdi)
> movl $0, 12(%rdi)
> movl $7241, 16(%rdi)
> ...
>
> ideally we'd have sth like
>
> pxor %xmm0, %xmm0
> movups %xmm0, (%rdi)
> movaps array+16(%rip), %xmm0
> movups %xmm0, 16(%rdi)
> ...
>
> thus have the zeros written as immediates and the remaining pieces
> with load+stores.
>
> The by-pieces infrastructure eventually get's to see
>
> (mem/u/c:BLK (symbol_ref:DI ("array") [flags 0x2] <var_decl
> 0x7ffff7ff5b40 array>) [1 array+0 S64 A256])
>
> where the MEM_EXPR should provide a way to access the constant initializer.
>
> That said I do agree the current code is a bit premature optimization
> - but maybe
> it should be fend off in expand_constructor which has the cheap clear_storage
> first and which already does check can_move_by_pieces with some heuristics,
> but that seems to be guarded by
>
> || (tree_fits_uhwi_p (TYPE_SIZE_UNIT (type))
> && (! can_move_by_pieces
> (tree_to_uhwi (TYPE_SIZE_UNIT (type)),
> TYPE_ALIGN (type)))
> && ! mostly_zeros_p (exp))))
>
> which is odd (we _can_ move by pieces, but how does this apply to
> TREE_CONSTANT CTORs and avoid_temp_mem?).
>
> That said, I wonder if we want to elide expand_constructor when the
> CTOR is TREE_STATIC && TREE_CONSTANT and !mostly_zeros_p
> and we can_move_by_pieces.
>
> So sth like
>
> diff --git a/gcc/expr.c b/gcc/expr.c
> index 7139545d543..76b3bdf0c01 100644
> --- a/gcc/expr.c
> +++ b/gcc/expr.c
> @@ -8504,6 +8504,12 @@ expand_constructor (tree exp, rtx target, enum
> expand_modifier modifier,
> && (! can_move_by_pieces
> (tree_to_uhwi (TYPE_SIZE_UNIT (type)),
> TYPE_ALIGN (type)))
> + && ! mostly_zeros_p (exp))
> + || (TREE_CONSTANT (exp)
> + && tree_fits_uhwi_p (TYPE_SIZE_UNIT (type))
> + && (can_move_by_pieces
> + (tree_to_uhwi (TYPE_SIZE_UNIT (type)),
> + TYPE_ALIGN (type)))
> && ! mostly_zeros_p (exp))))
> || ((modifier == EXPAND_INITIALIZER || modifier ==
> EXPAND_CONST_ADDRESS)
> && TREE_CONSTANT (exp)))
>
> which handles your initializer and the all-zero one optimal?
>
It works. Here is the updated patch.
Thanks.
--
H.J.
From 12989cce4d4c801e505dae96eb8fa36507382aa8 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.to...@gmail.com>
Date: Sun, 25 Apr 2021 13:56:32 -0700
Subject: [PATCH] constructor: Elide expand_constructor when can move by pieces
is true
Elide expand_constructor when
1. The constructor is TREE_STATIC && TREE_CONSTANT. And
2. mostly_zeros_p returns false. And
3. can_move_by_pieces returns true.
2021-XX-XX Richard Biener <rguent...@suse.de>
H.J. Lu <hjl.to...@gmail.com>
gcc/
PR middle-end/90773
* expr.c (expand_constructor): Elide expand_constructor when can
move by pieces is true.
gcc/testsuite/
PR middle-end/90773
* gcc.target/i386/pr90773-24.c: New test.
* gcc.target/i386/pr90773-25.c: Likewise.
* gcc.target/i386/pr90773-26.c: Likewise.
---
gcc/expr.c | 6 ++++++
gcc/testsuite/gcc.target/i386/pr90773-24.c | 23 ++++++++++++++++++++
gcc/testsuite/gcc.target/i386/pr90773-25.c | 21 ++++++++++++++++++
gcc/testsuite/gcc.target/i386/pr90773-26.c | 25 ++++++++++++++++++++++
4 files changed, 75 insertions(+)
create mode 100644 gcc/testsuite/gcc.target/i386/pr90773-24.c
create mode 100644 gcc/testsuite/gcc.target/i386/pr90773-25.c
create mode 100644 gcc/testsuite/gcc.target/i386/pr90773-26.c
diff --git a/gcc/expr.c b/gcc/expr.c
index d09ee42e262..068e429a296 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -8504,6 +8504,12 @@ expand_constructor (tree exp, rtx target, enum expand_modifier modifier,
&& (! can_move_by_pieces
(tree_to_uhwi (TYPE_SIZE_UNIT (type)),
TYPE_ALIGN (type)))
+ && ! mostly_zeros_p (exp))
+ || (TREE_CONSTANT (exp)
+ && tree_fits_uhwi_p (TYPE_SIZE_UNIT (type))
+ && (can_move_by_pieces
+ (tree_to_uhwi (TYPE_SIZE_UNIT (type)),
+ TYPE_ALIGN (type)))
&& ! mostly_zeros_p (exp))))
|| ((modifier == EXPAND_INITIALIZER || modifier == EXPAND_CONST_ADDRESS)
&& TREE_CONSTANT (exp)))
diff --git a/gcc/testsuite/gcc.target/i386/pr90773-24.c b/gcc/testsuite/gcc.target/i386/pr90773-24.c
new file mode 100644
index 00000000000..71f1fd8c4df
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr90773-24.c
@@ -0,0 +1,23 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=x86-64" } */
+
+struct S
+{
+ long long s1 __attribute__ ((aligned (8)));
+ unsigned s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14;
+};
+
+const struct S array[] = {
+ { 0, 60, 640, 2112543726, 39682, 48, 16, 33, 10, 96, 2, 0, 0, 4 }
+};
+
+void
+foo (struct S *x)
+{
+ x[0] = array[0];
+}
+
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, \\(%\[\^,\]+\\)" 1 } } */
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, 16\\(%\[\^,\]+\\)" 1 } } */
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, 32\\(%\[\^,\]+\\)" 1 } } */
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, 48\\(%\[\^,\]+\\)" 1 } } */
diff --git a/gcc/testsuite/gcc.target/i386/pr90773-25.c b/gcc/testsuite/gcc.target/i386/pr90773-25.c
new file mode 100644
index 00000000000..b2513c3a9c8
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr90773-25.c
@@ -0,0 +1,21 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=skylake" } */
+
+struct S
+{
+ long long s1 __attribute__ ((aligned (8)));
+ unsigned s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14;
+};
+
+const struct S array[] = {
+ { 0, 60, 640, 2112543726, 39682, 48, 16, 33, 10, 96, 2, 0, 0, 4 }
+};
+
+void
+foo (struct S *x)
+{
+ x[0] = array[0];
+}
+
+/* { dg-final { scan-assembler-times "vmovdqu\[\\t \]%ymm\[0-9\]+, \\(%\[\^,\]+\\)" 1 } } */
+/* { dg-final { scan-assembler-times "vmovdqu\[\\t \]%ymm\[0-9\]+, 32\\(%\[\^,\]+\\)" 1 } } */
diff --git a/gcc/testsuite/gcc.target/i386/pr90773-26.c b/gcc/testsuite/gcc.target/i386/pr90773-26.c
new file mode 100644
index 00000000000..ad19a88c883
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr90773-26.c
@@ -0,0 +1,25 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -march=x86-64" } */
+
+struct S
+{
+ long long s1 __attribute__ ((aligned (8)));
+ unsigned s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, s13, s14;
+};
+
+const struct S array[] = {
+ { 0, }
+};
+
+void
+foo (struct S *x)
+{
+ x[0] = array[0];
+}
+
+/* { dg-final { scan-assembler-not "movdqa" } } */
+/* { dg-final { scan-assembler-times "pxor\[\\t \]%xmm\[0-9\]+, %xmm\[0-9\]+" 1 } } */
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, \\(%\[\^,\]+\\)" 1 } } */
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, 16\\(%\[\^,\]+\\)" 1 } } */
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, 32\\(%\[\^,\]+\\)" 1 } } */
+/* { dg-final { scan-assembler-times "movups\[\\t \]%xmm\[0-9\]+, 48\\(%\[\^,\]+\\)" 1 } } */
--
2.31.1
