[Mesa-dev] [PATCH 1/2] gallivm: don't use control flow when doing indirect constant buffer lookups
Ilia Mirkin
imirkin at alum.mit.edu
Sat May 23 21:42:16 PDT 2015
Hi Roland,
I've just bisected a nv30 swtnl regression to this commit. When
running the nv30 driver (on a NV44, if it matters) and forcing swtnl
(NV30_SWTNL=1), glxgears from 60% broken to 100% broken. Now, I'm not
sure what the initial breakage is, but at least it shows a
gears-looking thing some of the time. After this change, I just see a
few random lines every 30 frames or so. Setting DRAW_USE_LLVM=0 makes
the whole thing fail horribly before and after this change (nothing
drawn at all).
Do you think that it's likely coincidence that the LLVM path worked at
all before this and that I should figure out what's not working with
DRAW_USE_LLVM=0? Or can you think of some reason why this change may
have broken the llvm path in a swtnl use-case with a driver like nv30
(no gs, no integers)?
Thanks,
-ilia
On Sat, Apr 4, 2015 at 10:50 AM, <sroland at vmware.com> wrote:
> From: Roland Scheidegger <sroland at vmware.com>
>
> llvm goes crazy when doing that, using way more memory and time, though there's
> probably more to it - this points to a very much similar issue as fixed in
> 8a9f5ecdb116d0449d63f7b94efbfa8b205d826f. In any case I've seen a quite
> plain looking vertex shader with just ~50 simple tgsi instructions (but with a
> dozen or so such indirect constant buffer lookups) go from a terribly high
> ~440ms compile time (consuming 25MB of memory in the process) down to a still
> awful ~230ms and 13MB with this fix (with llvm 3.3), so there's still obvious
> improvements possible (but I have no clue why it's so slow...).
> The resulting shader is most likely also faster (certainly seemed so though
> I don't have any hard numbers as it may have been influenced by compile times)
> since generally fetching constants outside the buffer range is most likely an
> app error (that is we expect all indices to be valid).
> It is possible this fixes some mysterious vertex shader slowdowns we've seen
> ever since we are conforming to newer apis at least partially (the main draw
> loop also has similar looking conditionals which we probably could do without -
> if not for the fetch at least for the additional elts condition.)
> ---
> src/gallium/auxiliary/draw/draw_llvm.h | 2 +
> .../draw/draw_pt_fetch_shade_pipeline_llvm.c | 27 +++---
> src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c | 95 +++++++++-------------
> src/gallium/drivers/llvmpipe/lp_scene.h | 2 +
> src/gallium/drivers/llvmpipe/lp_setup.c | 6 +-
> 5 files changed, 63 insertions(+), 69 deletions(-)
>
> diff --git a/src/gallium/auxiliary/draw/draw_llvm.h b/src/gallium/auxiliary/draw/draw_llvm.h
> index 9565fc6..a1983e1 100644
> --- a/src/gallium/auxiliary/draw/draw_llvm.h
> +++ b/src/gallium/auxiliary/draw/draw_llvm.h
> @@ -472,6 +472,8 @@ struct draw_llvm {
>
> struct draw_gs_llvm_variant_list_item gs_variants_list;
> int nr_gs_variants;
> +
> + float fake_const_buf[4];
> };
>
>
> diff --git a/src/gallium/auxiliary/draw/draw_pt_fetch_shade_pipeline_llvm.c b/src/gallium/auxiliary/draw/draw_pt_fetch_shade_pipeline_llvm.c
> index 0dfafdc..03257d8 100644
> --- a/src/gallium/auxiliary/draw/draw_pt_fetch_shade_pipeline_llvm.c
> +++ b/src/gallium/auxiliary/draw/draw_pt_fetch_shade_pipeline_llvm.c
> @@ -273,28 +273,35 @@ llvm_middle_end_bind_parameters(struct draw_pt_middle_end *middle)
> {
> struct llvm_middle_end *fpme = llvm_middle_end(middle);
> struct draw_context *draw = fpme->draw;
> + struct draw_llvm *llvm = fpme->llvm;
> unsigned i;
>
> - for (i = 0; i < Elements(fpme->llvm->jit_context.vs_constants); ++i) {
> + for (i = 0; i < Elements(llvm->jit_context.vs_constants); ++i) {
> int num_consts =
> draw->pt.user.vs_constants_size[i] / (sizeof(float) * 4);
> - fpme->llvm->jit_context.vs_constants[i] = draw->pt.user.vs_constants[i];
> - fpme->llvm->jit_context.num_vs_constants[i] = num_consts;
> + llvm->jit_context.vs_constants[i] = draw->pt.user.vs_constants[i];
> + llvm->jit_context.num_vs_constants[i] = num_consts;
> + if (num_consts == 0) {
> + llvm->jit_context.vs_constants[i] = llvm->fake_const_buf;
> + }
> }
> - for (i = 0; i < Elements(fpme->llvm->gs_jit_context.constants); ++i) {
> + for (i = 0; i < Elements(llvm->gs_jit_context.constants); ++i) {
> int num_consts =
> draw->pt.user.gs_constants_size[i] / (sizeof(float) * 4);
> - fpme->llvm->gs_jit_context.constants[i] = draw->pt.user.gs_constants[i];
> - fpme->llvm->gs_jit_context.num_constants[i] = num_consts;
> + llvm->gs_jit_context.constants[i] = draw->pt.user.gs_constants[i];
> + llvm->gs_jit_context.num_constants[i] = num_consts;
> + if (num_consts == 0) {
> + llvm->gs_jit_context.constants[i] = llvm->fake_const_buf;
> + }
> }
>
> - fpme->llvm->jit_context.planes =
> + llvm->jit_context.planes =
> (float (*)[DRAW_TOTAL_CLIP_PLANES][4]) draw->pt.user.planes[0];
> - fpme->llvm->gs_jit_context.planes =
> + llvm->gs_jit_context.planes =
> (float (*)[DRAW_TOTAL_CLIP_PLANES][4]) draw->pt.user.planes[0];
>
> - fpme->llvm->jit_context.viewports = draw->viewports;
> - fpme->llvm->gs_jit_context.viewports = draw->viewports;
> + llvm->jit_context.viewports = draw->viewports;
> + llvm->gs_jit_context.viewports = draw->viewports;
> }
>
>
> diff --git a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
> index 17b68ff..5aa2846 100644
> --- a/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
> +++ b/src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c
> @@ -944,20 +944,39 @@ gather_outputs(struct lp_build_tgsi_soa_context * bld)
> * with a little work.
> */
> static LLVMValueRef
> -build_gather(struct lp_build_context *bld,
> +build_gather(struct lp_build_tgsi_context *bld_base,
> LLVMValueRef base_ptr,
> LLVMValueRef indexes,
> - LLVMValueRef *overflow_mask)
> + LLVMValueRef overflow_mask)
> {
> - LLVMBuilderRef builder = bld->gallivm->builder;
> + struct gallivm_state *gallivm = bld_base->base.gallivm;
> + LLVMBuilderRef builder = gallivm->builder;
> + struct lp_build_context *uint_bld = &bld_base->uint_bld;
> + struct lp_build_context *bld = &bld_base->base;
> LLVMValueRef res = bld->undef;
> unsigned i;
> - LLVMValueRef temp_ptr = NULL;
> +
> + /*
> + * overflow_mask is a vector telling us which channels
> + * in the vector overflowed. We use the overflow behavior for
> + * constant buffers which is defined as:
> + * Out of bounds access to constant buffer returns 0 in all
> + * components. Out of bounds behavior is always with respect
> + * to the size of the buffer bound at that slot.
> + */
>
> if (overflow_mask) {
> - temp_ptr = lp_build_alloca(
> - bld->gallivm,
> - lp_build_vec_type(bld->gallivm, bld->type), "");
> + /*
> + * We avoid per-element control flow here (also due to llvm going crazy,
> + * though I suspect it's better anyway since overflow is likely rare).
> + * Note that since we still fetch from buffers even if num_elements was
> + * zero (in this case we'll fetch from index zero) the jit func callers
> + * MUST provide valid fake constant buffers of size 4x32 (the values do
> + * not matter), otherwise we'd still need (not per element though)
> + * control flow.
> + */
> + LLVMValueRef zeros = lp_build_const_int_vec(gallivm, uint_bld->type, 0);
> + indexes = lp_build_select(uint_bld, overflow_mask, zeros, indexes);
> }
>
> /*
> @@ -968,53 +987,17 @@ build_gather(struct lp_build_context *bld,
> LLVMValueRef index = LLVMBuildExtractElement(builder,
> indexes, ii, "");
> LLVMValueRef scalar_ptr, scalar;
> - LLVMValueRef overflow;
> - struct lp_build_if_state if_ctx;
> -
> - /*
> - * overflow_mask is a boolean vector telling us which channels
> - * in the vector overflowed. We use the overflow behavior for
> - * constant buffers which is defined as:
> - * Out of bounds access to constant buffer returns 0 in all
> - * componenets. Out of bounds behavior is always with respect
> - * to the size of the buffer bound at that slot.
> - */
> - if (overflow_mask) {
> - overflow = LLVMBuildExtractElement(builder, *overflow_mask,
> - ii, "");
> - lp_build_if(&if_ctx, bld->gallivm, overflow);
> - {
> - LLVMValueRef val = LLVMBuildLoad(builder, temp_ptr, "");
> - val = LLVMBuildInsertElement(
> - builder, val,
> - LLVMConstNull(LLVMFloatTypeInContext(bld->gallivm->context)),
> - ii, "");
> - LLVMBuildStore(builder, val, temp_ptr);
> - }
> - lp_build_else(&if_ctx);
> - {
> - LLVMValueRef val = LLVMBuildLoad(builder, temp_ptr, "");
> -
> - scalar_ptr = LLVMBuildGEP(builder, base_ptr,
> - &index, 1, "gather_ptr");
> - scalar = LLVMBuildLoad(builder, scalar_ptr, "");
> -
> - val = LLVMBuildInsertElement(builder, val, scalar, ii, "");
>
> - LLVMBuildStore(builder, val, temp_ptr);
> - }
> - lp_build_endif(&if_ctx);
> - } else {
> - scalar_ptr = LLVMBuildGEP(builder, base_ptr,
> - &index, 1, "gather_ptr");
> - scalar = LLVMBuildLoad(builder, scalar_ptr, "");
> + scalar_ptr = LLVMBuildGEP(builder, base_ptr,
> + &index, 1, "gather_ptr");
> + scalar = LLVMBuildLoad(builder, scalar_ptr, "");
>
> - res = LLVMBuildInsertElement(builder, res, scalar, ii, "");
> - }
> + res = LLVMBuildInsertElement(builder, res, scalar, ii, "");
> }
>
> if (overflow_mask) {
> - res = LLVMBuildLoad(builder, temp_ptr, "gather_val");
> + LLVMValueRef zeros = lp_build_const_vec(gallivm, bld->type, 0.0);
> + res = lp_build_select(bld, overflow_mask, zeros, res);
> }
>
> return res;
> @@ -1247,17 +1230,15 @@ emit_fetch_constant(
> num_consts = lp_build_broadcast_scalar(uint_bld, num_consts);
> /* Construct a boolean vector telling us which channels
> * overflow the bound constant buffer */
> - overflow_mask = LLVMBuildICmp(builder, LLVMIntUGE,
> - indirect_index,
> - num_consts, "");
> + overflow_mask = lp_build_compare(gallivm, uint_bld->type, PIPE_FUNC_GEQUAL,
> + indirect_index, num_consts);
>
> /* index_vec = indirect_index * 4 + swizzle */
> index_vec = lp_build_shl_imm(uint_bld, indirect_index, 2);
> index_vec = lp_build_add(uint_bld, index_vec, swizzle_vec);
>
> /* Gather values from the constant buffer */
> - res = build_gather(&bld_base->base, consts_ptr, index_vec,
> - &overflow_mask);
> + res = build_gather(bld_base, consts_ptr, index_vec, overflow_mask);
> }
> else {
> LLVMValueRef index; /* index into the const buffer */
> @@ -1319,7 +1300,7 @@ emit_fetch_immediate(
> FALSE);
>
> /* Gather values from the immediate register array */
> - res = build_gather(&bld_base->base, imms_array, index_vec, NULL);
> + res = build_gather(bld_base, imms_array, index_vec, NULL);
> } else {
> LLVMValueRef lindex = lp_build_const_int32(gallivm,
> reg->Register.Index * 4 + swizzle);
> @@ -1373,7 +1354,7 @@ emit_fetch_input(
> inputs_array = LLVMBuildBitCast(builder, bld->inputs_array, fptr_type, "");
>
> /* Gather values from the input register array */
> - res = build_gather(&bld_base->base, inputs_array, index_vec, NULL);
> + res = build_gather(bld_base, inputs_array, index_vec, NULL);
> } else {
> if (bld->indirect_files & (1 << TGSI_FILE_INPUT)) {
> LLVMValueRef lindex = lp_build_const_int32(gallivm,
> @@ -1495,7 +1476,7 @@ emit_fetch_temporary(
> temps_array = LLVMBuildBitCast(builder, bld->temps_array, fptr_type, "");
>
> /* Gather values from the temporary register array */
> - res = build_gather(&bld_base->base, temps_array, index_vec, NULL);
> + res = build_gather(bld_base, temps_array, index_vec, NULL);
> }
> else {
> LLVMValueRef temp_ptr;
> diff --git a/src/gallium/drivers/llvmpipe/lp_scene.h b/src/gallium/drivers/llvmpipe/lp_scene.h
> index ad23c20..3ef0f32 100644
> --- a/src/gallium/drivers/llvmpipe/lp_scene.h
> +++ b/src/gallium/drivers/llvmpipe/lp_scene.h
> @@ -178,6 +178,8 @@ struct lp_scene {
>
> struct cmd_bin tile[TILES_X][TILES_Y];
> struct data_block_list data;
> +
> + float fake_const_buf[4];
> };
>
>
> diff --git a/src/gallium/drivers/llvmpipe/lp_setup.c b/src/gallium/drivers/llvmpipe/lp_setup.c
> index 3b0056c..bfe0af4 100644
> --- a/src/gallium/drivers/llvmpipe/lp_setup.c
> +++ b/src/gallium/drivers/llvmpipe/lp_setup.c
> @@ -1103,14 +1103,16 @@ try_update_scene_state( struct lp_setup_context *setup )
> setup->constants[i].stored_size = current_size;
> setup->constants[i].stored_data = stored;
> }
> + setup->fs.current.jit_context.constants[i] =
> + setup->constants[i].stored_data;
> }
> else {
> setup->constants[i].stored_size = 0;
> setup->constants[i].stored_data = NULL;
> + setup->fs.current.jit_context.constants[i] =
> + scene->fake_const_buf;
> }
>
> - setup->fs.current.jit_context.constants[i] =
> - setup->constants[i].stored_data;
> num_constants =
> setup->constants[i].stored_size / (sizeof(float) * 4);
> setup->fs.current.jit_context.num_constants[i] = num_constants;
> --
> 1.9.1
>
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