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gallivm: Fix performance regressions due to vector selects.

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LLVM often can't determine the mask elements are all ones/zeros, and
there doesn't seem to be a good way to hint that.

Thanks to Roland Scheidegger for spotting and analyzing the issue.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
This commit is contained in:
Jose Fonseca 2016-04-02 15:13:38 +01:00
parent 11c4e5b45c
commit b284f1f7f9
1 changed files with 18 additions and 22 deletions
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40
src/gallium/auxiliary/gallivm/lp_bld_logic.c
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@ -39,6 +39,7 @@
#include "lp_bld_type.h"
#include "lp_bld_const.h"
#include "lp_bld_swizzle.h"
#include "lp_bld_init.h"
#include "lp_bld_intr.h"
#include "lp_bld_debug.h"
@ -314,35 +315,30 @@ lp_build_select(struct lp_build_context *bld,
mask = LLVMBuildTrunc(builder, mask, LLVMInt1TypeInContext(lc), "");
res = LLVMBuildSelect(builder, mask, a, b, "");
}
else if (HAVE_LLVM >= 0x0303) {
else if (LLVMIsConstant(mask) ||
LLVMGetInstructionOpcode(mask) == LLVMSExt) {
/* Generate a vector select.
*
* Using vector selects would avoid emitting intrinsics, but they weren't
* properly supported yet for a long time.
*
* LLVM 3.3 appears to reliably support it.
*
* LLVM 3.1 supports it, but it yields buggy code (e.g. lp_blend_test).
*
* LLVM 3.0 includes experimental support provided the -promote-elements
* options is passed to LLVM's command line (e.g., via
* llvm::cl::ParseCommandLineOptions), but resulting code quality is much
* worse, probably because some optimization passes don't know how to
* handle vector selects.
*
* See also:
* - http://lists.cs.uiuc.edu/pipermail/llvmdev/2011-October/043659.html
* Using vector selects should avoid emitting intrinsics hence avoid
* hidering optimization passes, but vector selects weren't properly
* supported yet for a long time, and LLVM will generate poor code when
* the mask is not the result of a comparison.
*/
/* Convert the mask to a vector of booleans.
* XXX: There are two ways to do this. Decide what's best.
*
* XXX: In x86 the mask is controlled by the MSB, so if we shifted the
* mask by `type.width - 1`, LLVM should realize the mask is ready. Alas
* what really happens is that LLVM will emit two shifts back to back.
*/
if (1) {
LLVMTypeRef bool_vec_type = LLVMVectorType(LLVMInt1TypeInContext(lc), type.length);
mask = LLVMBuildTrunc(builder, mask, bool_vec_type, "");
} else {
mask = LLVMBuildICmp(builder, LLVMIntNE, mask, LLVMConstNull(bld->int_vec_type), "");
if (0) {
LLVMValueRef shift = LLVMConstInt(bld->int_elem_type, bld->type.width - 1, 0);
shift = lp_build_broadcast(bld->gallivm, bld->int_vec_type, shift);
mask = LLVMBuildLShr(builder, mask, shift, "");
}
LLVMTypeRef bool_vec_type = LLVMVectorType(LLVMInt1TypeInContext(lc), type.length);
mask = LLVMBuildTrunc(builder, mask, bool_vec_type, "");
res = LLVMBuildSelect(builder, mask, a, b, "");
}
else if (((util_cpu_caps.has_sse4_1 &&