This clean-up patch resolves PR testsuite/104732, the failure of the recent
test gcc.target/i386/pr100711-1.c on 32-bit Solaris/x86. Rather than just
tweak the testcase, the proposed approach is to fix the underlying problem
by removing the "TARGET_STV && TARGET_SSE2" conditionals from the DI mode
logical operation expanders and pre-reload splitters in i386.md, which as
I'll show generate inferior code (even a GCC 12 regression) on !TARGET_64BIT
whenever -mno-stv (such as Solaris) or -msse (but not -msse2).
First a little bit of history. In the beginning, DImode operations on
i386 weren't defined by the machine description, and lowered during RTL
expansion to SI mode operations. The with PR 65105 in 2015, -mstv was
added, together with a SWIM1248x mode iterator (later renamed to SWIM1248x)
together with several *<code>di3_doubleword post-reload splitters that
made use of register allocation to perform some double word operations
in 64-but XMM registers. A short while later in 2016, PR 70322 added
similar support for one_cmpldi2. All of this logic was dependent upon
"!TARGET_64BIT && TARGET_STV && TARGET_SSE2". With the passing of time,
these conditions became irrelevant when in 2019, it was decided to split
these double-word patterns before reload.
https://gcc.gnu.org/pipermail/gcc-patches/2019-June/523877.htmlhttps://gcc.gnu.org/pipermail/gcc-patches/2019-October/532236.html
Hence the current situation, where on most modern CPU architectures
(where "TARGET_STV && TARGET_SSE2" is true), RTL is expanded with DI
mode operations, that are then split into two SI mode instructions
before reload, except on Solaris and other odd cases, where the splitting
is to two SI mode instructions is done during RTL expansion. By the
time compilation reaches register allocation both paths in theory
produce identical or similar code, so the vestigial legacy/logic would
appear to be harmless.
Unfortunately, there is one place where this arbitrary choice of how
to lower DI mode doubleword operations is visible to the middle-end,
it controls whether the backend appears to have a suitable optab, and
the presence (or not) of DImode optabs can influence vectorization
cost models and veclower decisions.
The issue (and code quality regression) can be seen in this test case:
typedef long long v2di __attribute__((vector_size (16)));
v2di x;
void foo (long long a)
{
v2di t = {a, a};
x = ~t;
}
which when compiled with "-O2 -m32 -msse -march=pentiumpro" produces:
foo: subl $28, %esp
movl %ebx, 16(%esp)
movl 32(%esp), %eax
movl %esi, 20(%esp)
movl 36(%esp), %edx
movl %edi, 24(%esp)
movl %eax, %esi
movl %eax, %edi
movl %edx, %ebx
movl %edx, %ecx
notl %esi
notl %ebx
movl %esi, (%esp)
notl %edi
notl %ecx
movl %ebx, 4(%esp)
movl 20(%esp), %esi
movl %edi, 8(%esp)
movl 16(%esp), %ebx
movl %ecx, 12(%esp)
movl 24(%esp), %edi
movss 8(%esp), %xmm1
movss 12(%esp), %xmm2
movss (%esp), %xmm0
movss 4(%esp), %xmm3
unpcklps %xmm2, %xmm1
unpcklps %xmm3, %xmm0
movlhps %xmm1, %xmm0
movaps %xmm0, x
addl $28, %esp
ret
Importantly notice the four "notl" instructions. With this patch:
foo: subl $28, %esp
movl 32(%esp), %edx
movl 36(%esp), %eax
notl %edx
movl %edx, (%esp)
notl %eax
movl %eax, 4(%esp)
movl %edx, 8(%esp)
movl %eax, 12(%esp)
movaps (%esp), %xmm1
movaps %xmm1, x
addl $28, %esp
ret
Notice only two "notl" instructions. Checking with godbolt.org, GCC
generated 4 NOTs in GCC 4.x and 5.x, 2 NOTs between GCC 6.x and 9.x,
and regressed to 4 NOTs since GCC 10.x [which hopefully qualifies
this clean-up as suitable for stage 4].
Most significantly, this patch allows pr100711-1.c to pass with
-mno-stv, allowing pandn to be used with V2DImode on Solaris/x86.
Fingers-crossed this should reduce the number of discrepancies
encountered supporting Solaris/x86.
2022-03-05 Roger Sayle <roger@nextmovesoftware.com>
Uroš Bizjak <ubizjak@gmail.com>
gcc/ChangeLog
PR testsuite/104732
* config/i386/i386.md (SWIM1248x): Renamed from SWIM1248s.
Include DI mode unconditionally.
(*anddi3_doubleword): Remove && TARGET_STV && TARGET_SSE2 condition,
i.e. always split on !TARGET_64BIT.
(*<any_or>di3_doubleword): Likewise.
(*one_cmpldi2_doubleword): Likewise.
(and<mode>3 expander): Update to use SWIM1248x from SWIM1248s.
(<any_or><mode>3 expander): Likewise.
(one_cmpl<mode>2 expander): Likewise.
gcc/testsuite/ChangeLog
PR testsuite/104732
* gcc.target/i386/pr104732.c: New test case.
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