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d55e743cfe
eliminating the llvm.x86.sse2.loadl.pd intrinsic?), one shuffle optzn may be done (if shufps is better than pinsw, Evan, please review), and we already know about LICM of simple instructions. llvm-svn: 45407
707 lines
20 KiB
Plaintext
707 lines
20 KiB
Plaintext
//===---------------------------------------------------------------------===//
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// Random ideas for the X86 backend: SSE-specific stuff.
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//===---------------------------------------------------------------------===//
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- Consider eliminating the unaligned SSE load intrinsics, replacing them with
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unaligned LLVM load instructions.
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//===---------------------------------------------------------------------===//
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Expand libm rounding functions inline: Significant speedups possible.
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http://gcc.gnu.org/ml/gcc-patches/2006-10/msg00909.html
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//===---------------------------------------------------------------------===//
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When compiled with unsafemath enabled, "main" should enable SSE DAZ mode and
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other fast SSE modes.
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//===---------------------------------------------------------------------===//
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Think about doing i64 math in SSE regs.
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//===---------------------------------------------------------------------===//
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This testcase should have no SSE instructions in it, and only one load from
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a constant pool:
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double %test3(bool %B) {
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%C = select bool %B, double 123.412, double 523.01123123
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ret double %C
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}
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Currently, the select is being lowered, which prevents the dag combiner from
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turning 'select (load CPI1), (load CPI2)' -> 'load (select CPI1, CPI2)'
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The pattern isel got this one right.
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//===---------------------------------------------------------------------===//
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SSE doesn't have [mem] op= reg instructions. If we have an SSE instruction
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like this:
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X += y
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and the register allocator decides to spill X, it is cheaper to emit this as:
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Y += [xslot]
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store Y -> [xslot]
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than as:
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tmp = [xslot]
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tmp += y
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store tmp -> [xslot]
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..and this uses one fewer register (so this should be done at load folding
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time, not at spiller time). *Note* however that this can only be done
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if Y is dead. Here's a testcase:
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%.str_3 = external global [15 x sbyte] ; <[15 x sbyte]*> [#uses=0]
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implementation ; Functions:
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declare void %printf(int, ...)
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void %main() {
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build_tree.exit:
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br label %no_exit.i7
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no_exit.i7: ; preds = %no_exit.i7, %build_tree.exit
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%tmp.0.1.0.i9 = phi double [ 0.000000e+00, %build_tree.exit ], [ %tmp.34.i18, %no_exit.i7 ] ; <double> [#uses=1]
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%tmp.0.0.0.i10 = phi double [ 0.000000e+00, %build_tree.exit ], [ %tmp.28.i16, %no_exit.i7 ] ; <double> [#uses=1]
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%tmp.28.i16 = add double %tmp.0.0.0.i10, 0.000000e+00
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%tmp.34.i18 = add double %tmp.0.1.0.i9, 0.000000e+00
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br bool false, label %Compute_Tree.exit23, label %no_exit.i7
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Compute_Tree.exit23: ; preds = %no_exit.i7
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tail call void (int, ...)* %printf( int 0 )
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store double %tmp.34.i18, double* null
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ret void
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}
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We currently emit:
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.BBmain_1:
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xorpd %XMM1, %XMM1
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addsd %XMM0, %XMM1
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*** movsd %XMM2, QWORD PTR [%ESP + 8]
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*** addsd %XMM2, %XMM1
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*** movsd QWORD PTR [%ESP + 8], %XMM2
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jmp .BBmain_1 # no_exit.i7
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This is a bugpoint reduced testcase, which is why the testcase doesn't make
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much sense (e.g. its an infinite loop). :)
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//===---------------------------------------------------------------------===//
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SSE should implement 'select_cc' using 'emulated conditional moves' that use
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pcmp/pand/pandn/por to do a selection instead of a conditional branch:
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double %X(double %Y, double %Z, double %A, double %B) {
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%C = setlt double %A, %B
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%z = add double %Z, 0.0 ;; select operand is not a load
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%D = select bool %C, double %Y, double %z
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ret double %D
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}
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We currently emit:
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_X:
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subl $12, %esp
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xorpd %xmm0, %xmm0
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addsd 24(%esp), %xmm0
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movsd 32(%esp), %xmm1
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movsd 16(%esp), %xmm2
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ucomisd 40(%esp), %xmm1
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jb LBB_X_2
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LBB_X_1:
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movsd %xmm0, %xmm2
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LBB_X_2:
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movsd %xmm2, (%esp)
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fldl (%esp)
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addl $12, %esp
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ret
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//===---------------------------------------------------------------------===//
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It's not clear whether we should use pxor or xorps / xorpd to clear XMM
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registers. The choice may depend on subtarget information. We should do some
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more experiments on different x86 machines.
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//===---------------------------------------------------------------------===//
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Currently the x86 codegen isn't very good at mixing SSE and FPStack
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code:
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unsigned int foo(double x) { return x; }
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foo:
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subl $20, %esp
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movsd 24(%esp), %xmm0
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movsd %xmm0, 8(%esp)
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fldl 8(%esp)
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fisttpll (%esp)
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movl (%esp), %eax
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addl $20, %esp
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ret
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This will be solved when we go to a dynamic programming based isel.
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//===---------------------------------------------------------------------===//
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Lower memcpy / memset to a series of SSE 128 bit move instructions when it's
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feasible.
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//===---------------------------------------------------------------------===//
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Teach the coalescer to commute 2-addr instructions, allowing us to eliminate
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the reg-reg copy in this example:
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float foo(int *x, float *y, unsigned c) {
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float res = 0.0;
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unsigned i;
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for (i = 0; i < c; i++) {
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float xx = (float)x[i];
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xx = xx * y[i];
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xx += res;
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res = xx;
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}
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return res;
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}
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LBB_foo_3: # no_exit
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cvtsi2ss %XMM0, DWORD PTR [%EDX + 4*%ESI]
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mulss %XMM0, DWORD PTR [%EAX + 4*%ESI]
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addss %XMM0, %XMM1
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inc %ESI
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cmp %ESI, %ECX
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**** movaps %XMM1, %XMM0
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jb LBB_foo_3 # no_exit
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//===---------------------------------------------------------------------===//
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Codegen:
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if (copysign(1.0, x) == copysign(1.0, y))
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into:
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if (x^y & mask)
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when using SSE.
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//===---------------------------------------------------------------------===//
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Use movhps to update upper 64-bits of a v4sf value. Also movlps on lower half
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of a v4sf value.
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//===---------------------------------------------------------------------===//
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Better codegen for vector_shuffles like this { x, 0, 0, 0 } or { x, 0, x, 0}.
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Perhaps use pxor / xorp* to clear a XMM register first?
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//===---------------------------------------------------------------------===//
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How to decide when to use the "floating point version" of logical ops? Here are
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some code fragments:
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movaps LCPI5_5, %xmm2
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divps %xmm1, %xmm2
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mulps %xmm2, %xmm3
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mulps 8656(%ecx), %xmm3
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addps 8672(%ecx), %xmm3
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andps LCPI5_6, %xmm2
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andps LCPI5_1, %xmm3
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por %xmm2, %xmm3
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movdqa %xmm3, (%edi)
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movaps LCPI5_5, %xmm1
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divps %xmm0, %xmm1
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mulps %xmm1, %xmm3
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mulps 8656(%ecx), %xmm3
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addps 8672(%ecx), %xmm3
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andps LCPI5_6, %xmm1
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andps LCPI5_1, %xmm3
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orps %xmm1, %xmm3
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movaps %xmm3, 112(%esp)
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movaps %xmm3, (%ebx)
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Due to some minor source change, the later case ended up using orps and movaps
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instead of por and movdqa. Does it matter?
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//===---------------------------------------------------------------------===//
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X86RegisterInfo::copyRegToReg() returns X86::MOVAPSrr for VR128. Is it possible
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to choose between movaps, movapd, and movdqa based on types of source and
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destination?
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How about andps, andpd, and pand? Do we really care about the type of the packed
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elements? If not, why not always use the "ps" variants which are likely to be
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shorter.
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//===---------------------------------------------------------------------===//
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External test Nurbs exposed some problems. Look for
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__ZN15Nurbs_SSE_Cubic17TessellateSurfaceE, bb cond_next140. This is what icc
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emits:
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movaps (%edx), %xmm2 #59.21
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movaps (%edx), %xmm5 #60.21
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movaps (%edx), %xmm4 #61.21
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movaps (%edx), %xmm3 #62.21
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movl 40(%ecx), %ebp #69.49
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shufps $0, %xmm2, %xmm5 #60.21
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movl 100(%esp), %ebx #69.20
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movl (%ebx), %edi #69.20
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imull %ebp, %edi #69.49
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addl (%eax), %edi #70.33
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shufps $85, %xmm2, %xmm4 #61.21
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shufps $170, %xmm2, %xmm3 #62.21
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shufps $255, %xmm2, %xmm2 #63.21
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lea (%ebp,%ebp,2), %ebx #69.49
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negl %ebx #69.49
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lea -3(%edi,%ebx), %ebx #70.33
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shll $4, %ebx #68.37
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addl 32(%ecx), %ebx #68.37
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testb $15, %bl #91.13
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jne L_B1.24 # Prob 5% #91.13
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This is the llvm code after instruction scheduling:
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cond_next140 (0xa910740, LLVM BB @0xa90beb0):
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%reg1078 = MOV32ri -3
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%reg1079 = ADD32rm %reg1078, %reg1068, 1, %NOREG, 0
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%reg1037 = MOV32rm %reg1024, 1, %NOREG, 40
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%reg1080 = IMUL32rr %reg1079, %reg1037
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%reg1081 = MOV32rm %reg1058, 1, %NOREG, 0
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%reg1038 = LEA32r %reg1081, 1, %reg1080, -3
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%reg1036 = MOV32rm %reg1024, 1, %NOREG, 32
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%reg1082 = SHL32ri %reg1038, 4
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%reg1039 = ADD32rr %reg1036, %reg1082
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%reg1083 = MOVAPSrm %reg1059, 1, %NOREG, 0
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%reg1034 = SHUFPSrr %reg1083, %reg1083, 170
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%reg1032 = SHUFPSrr %reg1083, %reg1083, 0
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%reg1035 = SHUFPSrr %reg1083, %reg1083, 255
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%reg1033 = SHUFPSrr %reg1083, %reg1083, 85
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%reg1040 = MOV32rr %reg1039
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%reg1084 = AND32ri8 %reg1039, 15
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CMP32ri8 %reg1084, 0
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JE mbb<cond_next204,0xa914d30>
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Still ok. After register allocation:
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cond_next140 (0xa910740, LLVM BB @0xa90beb0):
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%EAX = MOV32ri -3
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%EDX = MOV32rm <fi#3>, 1, %NOREG, 0
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ADD32rm %EAX<def&use>, %EDX, 1, %NOREG, 0
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%EDX = MOV32rm <fi#7>, 1, %NOREG, 0
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%EDX = MOV32rm %EDX, 1, %NOREG, 40
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IMUL32rr %EAX<def&use>, %EDX
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%ESI = MOV32rm <fi#5>, 1, %NOREG, 0
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%ESI = MOV32rm %ESI, 1, %NOREG, 0
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MOV32mr <fi#4>, 1, %NOREG, 0, %ESI
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%EAX = LEA32r %ESI, 1, %EAX, -3
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%ESI = MOV32rm <fi#7>, 1, %NOREG, 0
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%ESI = MOV32rm %ESI, 1, %NOREG, 32
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%EDI = MOV32rr %EAX
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SHL32ri %EDI<def&use>, 4
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ADD32rr %EDI<def&use>, %ESI
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%XMM0 = MOVAPSrm %ECX, 1, %NOREG, 0
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%XMM1 = MOVAPSrr %XMM0
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SHUFPSrr %XMM1<def&use>, %XMM1, 170
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%XMM2 = MOVAPSrr %XMM0
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SHUFPSrr %XMM2<def&use>, %XMM2, 0
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%XMM3 = MOVAPSrr %XMM0
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SHUFPSrr %XMM3<def&use>, %XMM3, 255
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SHUFPSrr %XMM0<def&use>, %XMM0, 85
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%EBX = MOV32rr %EDI
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AND32ri8 %EBX<def&use>, 15
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CMP32ri8 %EBX, 0
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JE mbb<cond_next204,0xa914d30>
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This looks really bad. The problem is shufps is a destructive opcode. Since it
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appears as operand two in more than one shufps ops. It resulted in a number of
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copies. Note icc also suffers from the same problem. Either the instruction
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selector should select pshufd or The register allocator can made the two-address
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to three-address transformation.
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It also exposes some other problems. See MOV32ri -3 and the spills.
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//===---------------------------------------------------------------------===//
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25500
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LLVM is producing bad code.
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LBB_main_4: # cond_true44
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addps %xmm1, %xmm2
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subps %xmm3, %xmm2
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movaps (%ecx), %xmm4
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movaps %xmm2, %xmm1
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addps %xmm4, %xmm1
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addl $16, %ecx
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incl %edx
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cmpl $262144, %edx
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movaps %xmm3, %xmm2
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movaps %xmm4, %xmm3
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jne LBB_main_4 # cond_true44
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There are two problems. 1) No need to two loop induction variables. We can
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compare against 262144 * 16. 2) Known register coalescer issue. We should
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be able eliminate one of the movaps:
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addps %xmm2, %xmm1 <=== Commute!
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subps %xmm3, %xmm1
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movaps (%ecx), %xmm4
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movaps %xmm1, %xmm1 <=== Eliminate!
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addps %xmm4, %xmm1
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addl $16, %ecx
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incl %edx
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cmpl $262144, %edx
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movaps %xmm3, %xmm2
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movaps %xmm4, %xmm3
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jne LBB_main_4 # cond_true44
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//===---------------------------------------------------------------------===//
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Consider:
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__m128 test(float a) {
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return _mm_set_ps(0.0, 0.0, 0.0, a*a);
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}
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This compiles into:
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movss 4(%esp), %xmm1
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mulss %xmm1, %xmm1
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xorps %xmm0, %xmm0
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movss %xmm1, %xmm0
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ret
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Because mulss doesn't modify the top 3 elements, the top elements of
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xmm1 are already zero'd. We could compile this to:
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movss 4(%esp), %xmm0
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mulss %xmm0, %xmm0
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ret
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//===---------------------------------------------------------------------===//
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Here's a sick and twisted idea. Consider code like this:
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__m128 test(__m128 a) {
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float b = *(float*)&A;
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...
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return _mm_set_ps(0.0, 0.0, 0.0, b);
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}
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This might compile to this code:
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movaps c(%esp), %xmm1
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xorps %xmm0, %xmm0
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movss %xmm1, %xmm0
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ret
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Now consider if the ... code caused xmm1 to get spilled. This might produce
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this code:
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movaps c(%esp), %xmm1
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movaps %xmm1, c2(%esp)
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...
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xorps %xmm0, %xmm0
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movaps c2(%esp), %xmm1
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movss %xmm1, %xmm0
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ret
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However, since the reload is only used by these instructions, we could
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"fold" it into the uses, producing something like this:
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movaps c(%esp), %xmm1
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movaps %xmm1, c2(%esp)
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...
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movss c2(%esp), %xmm0
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ret
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... saving two instructions.
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The basic idea is that a reload from a spill slot, can, if only one 4-byte
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chunk is used, bring in 3 zeros the the one element instead of 4 elements.
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This can be used to simplify a variety of shuffle operations, where the
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elements are fixed zeros.
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//===---------------------------------------------------------------------===//
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For this:
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#include <emmintrin.h>
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void test(__m128d *r, __m128d *A, double B) {
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*r = _mm_loadl_pd(*A, &B);
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}
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We generates:
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subl $12, %esp
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movsd 24(%esp), %xmm0
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movsd %xmm0, (%esp)
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movl 20(%esp), %eax
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movapd (%eax), %xmm0
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movlpd (%esp), %xmm0
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movl 16(%esp), %eax
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movapd %xmm0, (%eax)
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addl $12, %esp
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ret
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icc generates:
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movl 4(%esp), %edx #3.6
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movl 8(%esp), %eax #3.6
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movapd (%eax), %xmm0 #4.22
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movlpd 12(%esp), %xmm0 #4.8
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movapd %xmm0, (%edx) #4.3
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ret #5.1
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So icc is smart enough to know that B is in memory so it doesn't load it and
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store it back to stack.
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This should be fixed by eliminating the llvm.x86.sse2.loadl.pd intrinsic,
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lowering it to a load+insertelement instead. Already match the load+shuffle
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as movlpd, so this should be easy. We already get optimal code for:
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define void @test2(<2 x double>* %r, <2 x double>* %A, double %B) {
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entry:
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%tmp2 = load <2 x double>* %A, align 16
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%tmp8 = insertelement <2 x double> %tmp2, double %B, i32 0
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store <2 x double> %tmp8, <2 x double>* %r, align 16
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ret void
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}
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//===---------------------------------------------------------------------===//
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__m128d test1( __m128d A, __m128d B) {
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return _mm_shuffle_pd(A, B, 0x3);
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}
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compiles to
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shufpd $3, %xmm1, %xmm0
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Perhaps it's better to use unpckhpd instead?
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unpckhpd %xmm1, %xmm0
|
|
|
|
Don't know if unpckhpd is faster. But it is shorter.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This code generates ugly code, probably due to costs being off or something:
|
|
|
|
define void @test(float* %P, <4 x float>* %P2 ) {
|
|
%xFloat0.688 = load float* %P
|
|
%tmp = load <4 x float>* %P2
|
|
%inFloat3.713 = insertelement <4 x float> %tmp, float 0.0, i32 3
|
|
store <4 x float> %inFloat3.713, <4 x float>* %P2
|
|
ret void
|
|
}
|
|
|
|
Generates:
|
|
|
|
_test:
|
|
movl 8(%esp), %eax
|
|
movaps (%eax), %xmm0
|
|
pxor %xmm1, %xmm1
|
|
movaps %xmm0, %xmm2
|
|
shufps $50, %xmm1, %xmm2
|
|
shufps $132, %xmm2, %xmm0
|
|
movaps %xmm0, (%eax)
|
|
ret
|
|
|
|
Would it be better to generate:
|
|
|
|
_test:
|
|
movl 8(%esp), %ecx
|
|
movaps (%ecx), %xmm0
|
|
xor %eax, %eax
|
|
pinsrw $6, %eax, %xmm0
|
|
pinsrw $7, %eax, %xmm0
|
|
movaps %xmm0, (%ecx)
|
|
ret
|
|
|
|
?
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Some useful information in the Apple Altivec / SSE Migration Guide:
|
|
|
|
http://developer.apple.com/documentation/Performance/Conceptual/
|
|
Accelerate_sse_migration/index.html
|
|
|
|
e.g. SSE select using and, andnot, or. Various SSE compare translations.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Add hooks to commute some CMPP operations.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Apply the same transformation that merged four float into a single 128-bit load
|
|
to loads from constant pool.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Floating point max / min are commutable when -enable-unsafe-fp-path is
|
|
specified. We should turn int_x86_sse_max_ss and X86ISD::FMIN etc. into other
|
|
nodes which are selected to max / min instructions that are marked commutable.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We should compile this:
|
|
#include <xmmintrin.h>
|
|
typedef union {
|
|
int i[4];
|
|
float f[4];
|
|
__m128 v;
|
|
} vector4_t;
|
|
void swizzle (const void *a, vector4_t * b, vector4_t * c) {
|
|
b->v = _mm_loadl_pi (b->v, (__m64 *) a);
|
|
c->v = _mm_loadl_pi (c->v, ((__m64 *) a) + 1);
|
|
}
|
|
|
|
to:
|
|
|
|
_swizzle:
|
|
movl 4(%esp), %eax
|
|
movl 8(%esp), %edx
|
|
movl 12(%esp), %ecx
|
|
movlps (%eax), %xmm0
|
|
movlps %xmm0, (%edx)
|
|
movlps 8(%eax), %xmm0
|
|
movlps %xmm0, (%ecx)
|
|
ret
|
|
|
|
not:
|
|
|
|
swizzle:
|
|
movl 8(%esp), %eax
|
|
movaps (%eax), %xmm0
|
|
movl 4(%esp), %ecx
|
|
movlps (%ecx), %xmm0
|
|
movaps %xmm0, (%eax)
|
|
movl 12(%esp), %eax
|
|
movaps (%eax), %xmm0
|
|
movlps 8(%ecx), %xmm0
|
|
movaps %xmm0, (%eax)
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
These functions should produce the same code:
|
|
|
|
#include <emmintrin.h>
|
|
|
|
typedef long long __m128i __attribute__ ((__vector_size__ (16)));
|
|
|
|
int foo(__m128i* val) {
|
|
return __builtin_ia32_vec_ext_v4si(*val, 1);
|
|
}
|
|
int bar(__m128i* val) {
|
|
union vs {
|
|
__m128i *_v;
|
|
int* _s;
|
|
} v = {val};
|
|
return v._s[1];
|
|
}
|
|
|
|
We currently produce (with -m64):
|
|
|
|
_foo:
|
|
pshufd $1, (%rdi), %xmm0
|
|
movd %xmm0, %eax
|
|
ret
|
|
_bar:
|
|
movl 4(%rdi), %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We should materialize vector constants like "all ones" and "signbit" with
|
|
code like:
|
|
|
|
cmpeqps xmm1, xmm1 ; xmm1 = all-ones
|
|
|
|
and:
|
|
cmpeqps xmm1, xmm1 ; xmm1 = all-ones
|
|
psrlq xmm1, 31 ; xmm1 = all 100000000000...
|
|
|
|
instead of using a load from the constant pool. The later is important for
|
|
ABS/NEG/copysign etc.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
"converting 64-bit constant pool entry to 32-bit not necessarily beneficial"
|
|
http://llvm.org/PR1264
|
|
|
|
For this test case:
|
|
|
|
define double @foo(double %x) {
|
|
%y = mul double %x, 5.000000e-01
|
|
ret double %y
|
|
}
|
|
|
|
llc -march=x86-64 currently produces a 32-bit constant pool entry and this code:
|
|
|
|
cvtss2sd .LCPI1_0(%rip), %xmm1
|
|
mulsd %xmm1, %xmm0
|
|
|
|
instead of just using a 64-bit constant pool entry with this:
|
|
|
|
mulsd .LCPI1_0(%rip), %xmm0
|
|
|
|
This is due to the code in ExpandConstantFP in LegalizeDAG.cpp. It notices that
|
|
x86-64 indeed has an instruction to load a 32-bit float from memory and convert
|
|
it into a 64-bit float in a register, however it doesn't notice that this isn't
|
|
beneficial because it prevents the load from being folded into the multiply.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
These functions:
|
|
|
|
#include <xmmintrin.h>
|
|
__m128i a;
|
|
void x(unsigned short n) {
|
|
a = _mm_slli_epi32 (a, n);
|
|
}
|
|
void y(unsigned n) {
|
|
a = _mm_slli_epi32 (a, n);
|
|
}
|
|
|
|
compile to ( -O3 -static -fomit-frame-pointer):
|
|
_x:
|
|
movzwl 4(%esp), %eax
|
|
movd %eax, %xmm0
|
|
movaps _a, %xmm1
|
|
pslld %xmm0, %xmm1
|
|
movaps %xmm1, _a
|
|
ret
|
|
_y:
|
|
movd 4(%esp), %xmm0
|
|
movaps _a, %xmm1
|
|
pslld %xmm0, %xmm1
|
|
movaps %xmm1, _a
|
|
ret
|
|
|
|
"y" looks good, but "x" does silly movzwl stuff around into a GPR. It seems
|
|
like movd would be sufficient in both cases as the value is already zero
|
|
extended in the 32-bit stack slot IIRC. For signed short, it should also be
|
|
save, as a really-signed value would be undefined for pslld.
|
|
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
#include <math.h>
|
|
int t1(double d) { return signbit(d); }
|
|
|
|
This currently compiles to:
|
|
subl $12, %esp
|
|
movsd 16(%esp), %xmm0
|
|
movsd %xmm0, (%esp)
|
|
movl 4(%esp), %eax
|
|
shrl $31, %eax
|
|
addl $12, %esp
|
|
ret
|
|
|
|
We should use movmskp{s|d} instead.
|
|
|