On Thu, May 20, 2021 at 11:57 PM Richard Biener
<richard.guent...@gmail.com> wrote:
>
> On Thu, May 20, 2021 at 4:04 PM H.J. Lu <hjl.to...@gmail.com> wrote:
> >
> > 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.
>
> So just looking at the code again I think we probably want to add
> && avoid_temp_mem here, at least that's the case we're looking
> at. Not sure if we ever arrive with TREE_CONSTANT CTORs
> and !avoid_temp_mem but if so we'd create a temporary here
> which of course would be pointless.
>
> So maybe it's then clearer to split the condition out as
>
> diff --git a/gcc/expr.c b/gcc/expr.c
> index 7139545d543..ee8f25f9abd 100644
> --- a/gcc/expr.c
> +++ b/gcc/expr.c
> @@ -8523,6 +8523,19 @@ expand_constructor (tree exp, rtx target, enum
> expand_modifier modifier,
> return constructor;
> }
>
> + /* If the CTOR is available in static storage and not mostly
> + zeros and we can move it by pieces prefer to do so since
> + that's usually more efficient than performing a series of
> + stores from immediates. */
> + if (avoid_temp_mem
> + && TREE_STATIC (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))
> + return NULL_RTX;
> +
> /* Handle calls that pass values in multiple non-contiguous
> locations. The Irix 6 ABI has examples of this. */
> if (target == 0 || ! safe_from_p (target, exp, 1)
>
>
> OK with that change.
>
This is the patch I am checking in after testing.
Thanks.
--
H.J.
From ca3e15ff233230f2450478459a41e6a93ec42df6 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.to...@gmail.com>
Date: Fri, 21 May 2021 05:16:20 -0700
Subject: [PATCH] Elide expand_constructor if move by pieces is preferred
Elide expand_constructor when the constructor is static storage and not
mostly zeros and we can move it by pieces prefer to do so since that's
usually more efficient than performing a series of stores from immediates.
2021-05-21 Richard Biener <rguent...@suse.de>
H.J. Lu <hjl.to...@gmail.com>
gcc/
PR middle-end/90773
* expr.c (expand_constructor): Elide expand_constructor if
move by pieces is preferred.
gcc/testsuite/
* gcc.target/i386/pr90773-24.c: New test.
* gcc.target/i386/pr90773-25.c: Likewise.
---
gcc/expr.c | 13 +++++++++++
gcc/testsuite/gcc.target/i386/pr90773-24.c | 23 ++++++++++++++++++++
gcc/testsuite/gcc.target/i386/pr90773-25.c | 25 ++++++++++++++++++++++
3 files changed, 61 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..ba61eb98b3b 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -8523,6 +8523,19 @@ expand_constructor (tree exp, rtx target, enum expand_modifier modifier,
return constructor;
}
+ /* If the CTOR is available in static storage and not mostly
+ zeros and we can move it by pieces prefer to do so since
+ that's usually more efficient than performing a series of
+ stores from immediates. */
+ if (avoid_temp_mem
+ && TREE_STATIC (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))
+ return NULL_RTX;
+
/* Handle calls that pass values in multiple non-contiguous
locations. The Irix 6 ABI has examples of this. */
if (target == 0 || ! safe_from_p (target, exp, 1)
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..7b2ea66dcfc
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr90773-24.c
@@ -0,0 +1,23 @@
+/* { dg-do compile { target { ! ia32 } } } */
+/* { 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..57642ea8d2d
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr90773-25.c
@@ -0,0 +1,25 @@
+/* { dg-do compile { target { ! ia32 } } } */
+/* { 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
