https://llvm.org/bugs/show_bug.cgi?id=26810
Bug ID: 26810
Summary: Redundant movs and an extra spill while using all xmm
registers
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Register Allocator
Assignee: unassignedb...@nondot.org
Reporter: egor.koche...@intel.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 15974
--> https://llvm.org/bugs/attachment.cgi?id=15974&action=edit
The reproducer (1 source file, assemblies for clang and GCC, auxiliary
Makefile)
When compiling the loop below, clang produces the binary that runs ~25% slower
than that of GCC.
constexpr unsigned nIterations = 76800000; //< an arbitrary large number
__m128d v[8];
__m128d m1[nIterations];
__m128d m2[8];
for (unsigned i = 0; i < nIterations; ++i) {
const __m128d M = m1 [i];
__m128d a;
a = _mm_mul_pd(M, m2[0]);
v[0] = _mm_add_pd(v[0], a);
a = _mm_mul_pd(M, m2[1]);
v[1] = _mm_add_pd(v[1], a);
a = _mm_mul_pd(M, m2[2]);
v[2] = _mm_add_pd(v[2], a);
a = _mm_mul_pd(M, m2[3]);
v[3] = _mm_add_pd(v[3], a);
a = _mm_mul_pd(M, m2[4]);
v[4] = _mm_add_pd(v[4], a);
a = _mm_mul_pd(M, m2[5]);
v[5] = _mm_add_pd(v[5], a);
a = _mm_mul_pd(M, m2[6]);
v[6] = _mm_add_pd(v[6], a);
a = _mm_mul_pd(M, m2[7]);
v[7] = _mm_add_pd(v[7], a);
}
In particular, there are weird movs around calculating v[6]:
0x08048c4e <+334>: movapd %xmm5,%xmm6
0x08048c52 <+338>: movapd %xmm4,%xmm5
0x08048c56 <+342>: movapd %xmm3,%xmm4
0x08048c5a <+346>: movapd %xmm2,%xmm3
Also, throughout the loop clang uses 3 spills for storing values of v[1], v[6]
and v[7], while GCC uses only two spills (for v[0] and v[1]).
Attached is the source file, annotated assembly for clang and an assembly for
GCC for the function 'loop', and the (optional) Makefile.
The source needs to be compiled with '-m32' to limit the number of xmm
registers available. -O2 and -Ofast produce the same results (though the
register names are changing).
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