[llvm-dev] RFC: A proposal for vectorizing loops with calls to math functions using SVML
Sanjay Patel via llvm-dev
llvm-dev at lists.llvm.org
Tue Apr 5 07:25:27 PDT 2016
Thanks, Matt. I was just curious if the existing hook would support another
lib or if more infrastructure was needed.
Since this proposal can build on the existing code, adds functionality, and
increases compatibility with other compilers, I'm assuming patches would be
accepted, but you probably want to get the green light from someone more
familiar with the vectorizers and/or veclibs.
On Mon, Apr 4, 2016 at 5:39 PM, Masten, Matt <matt.masten at intel.com> wrote:
> Hi Sanjay,
>
>
>
> For sincos calls, I’m currently just going through
> isTriviallyVectorizable(), which was good enough to get things working so
> that I could test the translation. I don’t see why this cannot be changed
> to use addVectorizableFunctionsFromVecLib(). The other functions that I’m
> working with are already vectorized using the loop pragma. Those include
> sin, cos, exp, log, and pow.
>
>
>
> *From:* Sanjay Patel [mailto:spatel at rotateright.com]
> *Sent:* Monday, April 04, 2016 10:57 AM
> *To:* Masten, Matt
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] RFC: A proposal for vectorizing loops with
> calls to math functions using SVML
>
>
>
> Hi Matt -
>
> Are you using the same TLI hook as Darwin's Accelerate framework:
> addVectorizableFunctionsFromVecLib()? If not, why not?
>
>
>
> On Thu, Mar 31, 2016 at 6:20 PM, Masten, Matt via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> RFC: A proposal for vectorizing loops with calls to math functions using
> SVML (short
> vector math library).
>
> =========
> Overview
> =========
>
> Very simply, SVML (Intel short vector math library) functions are vector
> variants of
> scalar math functions that take vector arguments, apply an operation to
> each
> element, and store the result in a vector register. These vector variants
> can be
> generated by the compiler, based on precision requirements specified by the
> user, resulting in substantial performance gains. This is an initial
> proposal to
> add a new LLVM IR transformation pass that will translate scalar math
> calls to
> svml calls with the help of the loop vectorizer.
>
> ====================
> Problem Description
> ====================
>
> Currently, without the "#pragma clang loop vectorize(enable)", the loop
> vectorizer will not vectorize loops with math calls due to cost model
> reasons.
> Additionally, When the loop pragma is used, the loop vectorizer will widen
> the
> math call using an intrinsic, but the resulting code is inefficient
> because the
> intrinsic is replaced with scalarized function calls. Please see the
> example
> below for a simple loop containing a sinf call. For demonstration
> purposes, the
> example was compiled for an xmm target, thus VF = 4 given the float type.
>
> Example: sinf.c
>
> #define N 1000
>
> #pragma clang loop vectorize(enable)
> for (i = 0; i < N; i++) {
> array[i] = sinf((float)i);
> }
>
> Without the loop pragma the loop vectorizer's cost model rejects the loop.
>
> clang -c -ffast-math -O2 -Rpass-analysis=loop-vectorize
> -Rpass-missed=loop-vectorize sinf.c
>
> sinf.c:19:3: remark: the cost-model indicates that vectorization is not
> beneficial [-Rpass-analysis=loop-vectorize]
> for (i = 0; i < N; i++) {
> ^
> sinf.c:19:3: remark: the cost-model indicates that interleaving is not
> beneficial and is explicitly disabled or interleave count is set to 1
> [-Rpass-analysis=loop-vectorize]
>
> When the the loop pragma is used, the loop is vectorized and the call to
> @llvm.sin.v4f32 is generated, but the call is later scalarized with the
> additional overhead of unpacking the scalar function arguments from a
> vector.
> This can be seen from inspection of the resulting assembly code just below
> the
> LLVM IR.
>
> vector.body: ; preds = %vector.body, %
> vector.ph
> %index = phi i64 [ 0, %vector.ph ], [ %index.next, %vector.body ], !dbg
> !6
> %0 = trunc i64 %index to i32, !dbg !7
> %broadcast.splatinsert6 = insertelement <4 x i32> undef, i32 %0, i32 0,
> !dbg !7
> %broadcast.splat7 = shufflevector <4 x i32> %broadcast.splatinsert6,
> <4 x i32> undef, <4 x i32> zeroinitializer, !dbg !7
> %induction8 = add <4 x i32> %broadcast.splat7, <i32 0, i32 1, i32 2, i32
> 3>,
> !dbg !7
> %1 = sitofp <4 x i32> %induction8 to <4 x float>, !dbg !7
> %2 = call <4 x float> @llvm.sin.v4f32(<4 x float> %1), !dbg !8
> %3 = getelementptr inbounds float, float* %array, i64 %index, !dbg !9
> %4 = bitcast float* %3 to <4 x float>*, !dbg !10
> store <4 x float> %2, <4 x float>* %4, align 4, !dbg !10, !tbaa !11
> %index.next = add i64 %index, 4, !dbg !6
> %5 = icmp eq i64 %index.next, 1000, !dbg !6
> br i1 %5, label %middle.block, label %vector.body, !dbg !6, !llvm.loop
> !15
>
>
> .LBB0_1: # %vector.body
> # =>This Inner Loop Header: Depth=1
> movd %ebx, %xmm0
> pshufd $0, %xmm0, %xmm0 # xmm0 = xmm0[0,0,0,0]
> paddd .LCPI0_0(%rip), %xmm0
> cvtdq2ps %xmm0, %xmm0
> movaps %xmm0, 16(%rsp) # 16-byte Spill
> shufps $231, %xmm0, %xmm0 # xmm0 = xmm0[3,1,2,3]
> callq sinf
> movaps %xmm0, (%rsp) # 16-byte Spill
> movaps 16(%rsp), %xmm0 # 16-byte Reload
> shufps $229, %xmm0, %xmm0 # xmm0 = xmm0[1,1,2,3]
> callq sinf
> unpcklps (%rsp), %xmm0 # 16-byte Folded Reload
> # xmm0 =
> xmm0[0],mem[0],xmm0[1],mem[1]
> movaps %xmm0, (%rsp) # 16-byte Spill
> movaps 16(%rsp), %xmm0 # 16-byte Reload
> callq sinf
> movaps %xmm0, 32(%rsp) # 16-byte Spill
> movapd 16(%rsp), %xmm0 # 16-byte Reload
> shufpd $1, %xmm0, %xmm0 # xmm0 = xmm0[1,0]
> callq sinf
> movaps 32(%rsp), %xmm1 # 16-byte Reload
> unpcklps %xmm0, %xmm1 # xmm1 =
> xmm1[0],xmm0[0],xmm1[1],xmm0[1]
> unpcklps (%rsp), %xmm1 # 16-byte Folded Reload
> # xmm1 =
> xmm1[0],mem[0],xmm1[1],mem[1]
> movups %xmm1, (%r14,%rbx,4)
> addq $4, %rbx
> cmpq $1000, %rbx # imm = 0x3E8
> jne .LBB0_1
>
> ===========================
> Proposed New Functionality
> ===========================
>
> In order to take advantage of the performance benefits of the svml
> library, the
> proposed solution is to introduce a new LLVM IR pass that is capable of
> translating the vector math intrinsics to svml calls. As an example, the
> LLVM IR
> above for "vector.body", introduced in the Problem Description section,
> would
> serve as input to the proposed pass and be transformed into the following
> LLVM
> IR. Special attention should be paid to the "__svml_sinf4_ha" call in the
> LLVM
> IR and resulting assembly code snippet.
>
> vector.body: ; preds = %vector.body,
> %entry
> %index = phi i64 [ 0, %entry ], [ %index.next, %vector.body ], !dbg !6
> %0 = trunc i64 %index to i32, !dbg !7
> %broadcast.splatinsert6 = insertelement <4 x i32> undef, i32 %0, i32 0,
> !dbg !7
> %broadcast.splat7 = shufflevector <4 x i32> %broadcast.splatinsert6,
> <4 x i32> undef, <4 x i32> zeroinitializer, !dbg !7
> %induction8 = add <4 x i32> %broadcast.splat7, <i32 0, i32 1, i32 2, i32
> 3>,
> !dbg !7
> %1 = sitofp <4 x i32> %induction8 to <4 x float>, !dbg !7
> %vcall = call <4 x float> @__svml_sinf4_ha(<4 x float> %1)
> %2 = getelementptr inbounds float, float* %array, i64 %index, !dbg !8
> %3 = bitcast float* %2 to <4 x float>*, !dbg !9
> store <4 x float> %vcall, <4 x float>* %3, align 4, !dbg !9, !tbaa !10
> %index.next = add i64 %index, 4, !dbg !6
> %4 = icmp eq i64 %index.next, 1000, !dbg !6
> br i1 %4, label %for.end, label %vector.body, !dbg !6, !llvm.loop !14
>
> The resulting assembly would appear as:
>
> .LBB0_1: # %vector.body
> # =>This Inner Loop Header: Depth=1
> movd %ebx, %xmm0
> pshufd $0, %xmm0, %xmm0 # xmm0 = xmm0[0,0,0,0]
> paddd .LCPI0_0(%rip), %xmm0
> cvtdq2ps %xmm0, %xmm0
> callq __svml_sinf4_ha
> movups %xmm0, (%r14,%rbx,4)
> addq $4, %rbx
> cmpq $1000, %rbx # imm = 0x3E8
> jne .LBB0_1
>
> In order to perform the translation, several requirements must be met to
> guide
> code generation. Those include:
>
> 1) In addition to the -ffast-math flag, support is needed from clang to
> allow
> the user to be able to specify the desired precision requirements. The
> additional flags needed include the following, where "imf" is shorthand
> for
> "Intel math function".
>
> -fimf-absolute-error=value[:funclist]
> define the maximum allowable absolute error for math library
> function results
> value - a positive, floating-point number conforming to the
> format [digits][.digits][{e|E}[sign]digits]
> funclist - optional comma separated list of one or more math
> library functions to which the attribute should be
> applied
>
> -fimf-accuracy-bits=bits[:funclist]
> define the relative error, measured by the number of correct
> bits,
> for math library function results
> bits - a positive, floating-point number
> funclist - optional comma separated list of one or more math
> library functions to which the attribute should be
> applied
>
> -fimf-arch-consistency=value[:funclist]
> ensures that the math library functions produce consistent
> results
> across different implementations of the same architecture
> value - true or false
> funclist - optional comma separated list of one or more math
> library functions to which the attribute should be
> applied
>
> -fimf-max-error=ulps[:funclist]
> defines the maximum allowable relative error, measured in ulps,
> for
> math library function results
> ulps - a positive, floating-point number conforming to the
> format [digits][.digits][{e|E}[sign]digits]
> funclist - optional comma separated list of one or more math
> library functions to which the attribute should be
> applied
>
> -fimf-precision=value[:funclist]
> defines the accuracy (precision) for math library functions
> value - defined as one of the following values
> high - equivalent to max-error = 0.6
> medium - equivalent to max-error = 4
> low - equivalent to accuracy-bits = 11 (single
> precision); accuracy-bits = 26 (double
> precision)
> funclist - optional comma separated list of one or more math
> library functions to which the attribute should be
> applied
>
> -fimf-domain-exclusion=classlist[:funclist]
> indicates the input arguments domain on which math functions
> must provide correct results.
> classlist - defined as one of the following values
> nans, infinities, denormals, zeros
> all, none, common
> funclist - optional list of one or more math library
> functions to which the attribute should be applied.
>
> Information from the flags can then be encoded as function attributes at
> each
> call site. In the future, this functionality will enable more fine-grained
> control over specifying precision for individual calls/regions, instead of
> setting the precision requirements for all call instances of a function.
> Please
> note that the example translation presented so far does not have the IMF
> attributes attached to the @llvm.sin.v4f32 call, and as a result the
> default is
> set to an svml variant marked with "_ha" (max-error = 0.6), which is short
> for
> high accuracy. Other supported variants will include low precision,
> enhanced
> performance, bitwise reproducible, and correctly rounded. Please refer to
> the
> IEEE-754 standard for additional information regarding supported
> precisions.
> The compiler will select the most appropriate variant based on the IMF
> attributes. See #2.
>
> 2) An interface to query for the appropriate svml function variant based
> on the
> scalar function name and IMF attributes.
>
> 3) For calls to math functions that store to memory (e.g., sincos),
> additional
> analysis of the pointer arguments is beneficial in order to generate
> the best
> performing load/store instructions.
>
> ======================
> GCC/ICC compatibility
> ======================
>
> The initial implementation will involve the translation of 6 svml
> functions,
> which include sin, cos, log, pow, exp, and sincos (both single and double
> precision variants). Support for these functions matches the current
> capabilities of GCC and a subset of ICC. As more functions become
> open-sourced,
> the plan is to support them as part of the final solution determined from
> this
> proposal. The flags referenced in the Proposed New Functionality section
> are
> required to maintain icc compatibility.
>
> =======================
> Current Implementation
> =======================
>
> To evaluate the feasibility of this proposal, a prototype transform pass
> has
> been developed, which performs the following:
>
> 1) Searches for vector math intrinsics as candidates for translation to
> svml.
>
> 2) Reads function attributes to obtain precision requirements for the
> call. If
> none, default to attributes that will force the selection of a high
> accuracy
> variant.
>
> 3) Since the vector factor of the intrinsic can be wider than what is
> legally
> supported by the target, type legalization is performed so that the
> correct
> svml variant is selected. For example, if a call to
> @llvm.sin.v8f32(<8 x float> %1) is made for an xmm target, the pass will
> generate two __svml_sinf4 calls and will do the appropriate splitting
> of %1
> to create the new arguments for each call. In addition, the multiple
> return
> vectors are recombined and users of the original return vector are
> updated.
> The pass is also capable of handling less than full vector cases. E.g.,
> @llvm.sin.v2f32.
>
> 4) Special handling for sincos since the results are stored to a double
> wide
> vector and additional analysis is needed to optimize the stores to
> memory.
> Additional shuffling is required to obtain the sin and cos results from
> the double wide vector.
>
> 5) Vector intrinsics that are not translated to svml are scalarized.
>
> 6) The loop vectorizer has been taught to allow widening of sincos and
> additional utilities have been written to analyze arguments for sincos.
>
> =========
> Feedback
> =========
>
> For those who are interested in this topic, I would like to ask for your
> review
> of this proposal and would definitely appreciate any/all feedback on the
> proposed approach. Help is also very welcome and much appreciated in the
> development process.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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