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[AMDGPU] Add attribute for target loop unroll threshold default

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Summary:
Add a function attribute to allow the target specific default loop unroll threshold
to be specified on a per-function basis. This allows a front-end to give guidance
where it has insight that is not available to the back-end, while still allowing the
target specific heuristics to also have an effect.

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68873
This commit is contained in:
Tim Corringham 2019-10-11 16:55:39 +01:00
parent cb47dbbe79
commit 511fbac991
2 changed files with 54 additions and 1 deletions
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3
lib/Target/AMDGPU/AMDGPUTargetTransformInfo.cpp
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@ -90,7 +90,8 @@ static bool dependsOnLocalPhi(const Loop *L, const Value *Cond,
void AMDGPUTTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
TTI::UnrollingPreferences &UP) {
UP.Threshold = 300; // Twice the default.
const Function &F = *L->getHeader()->getParent();
UP.Threshold = AMDGPU::getIntegerAttribute(F, "amdgpu-unroll-threshold", 300);
UP.MaxCount = std::numeric_limits<unsigned>::max();
UP.Partial = true;

52
test/CodeGen/AMDGPU/amdgpu-unroll-threshold.ll Normal file
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@ -0,0 +1,52 @@
; RUN: opt < %s -S -mtriple=amdgcn-- -basicaa -loop-unroll | FileCheck %s
; Check that the loop in unroll_default is not fully unrolled using the default
; unroll threshold
; CHECK-LABEL: @unroll_default
; CHECK: entry:
; CHECK: br i1 %cmp
; CHECK: ret void
; Check that the same loop in unroll_full is fully unrolled when the default
; unroll threshold is increased by use of the amdgpu-unroll-threshold attribute
; CHECK-LABEL: @unroll_full
; CHECK: entry:
; CHECK-NOT: br i1 %cmp
; CHECK: ret void
@in = internal unnamed_addr global i32* null, align 8
@out = internal unnamed_addr global i32* null, align 8
define void @unroll_default() {
entry:
br label %do.body
do.body: ; preds = %entry
%i.0 = phi i32 [ 0, %entry ], [ %inc, %do.body ]
%v1 = load i64, i64* bitcast (i32** @in to i64*), align 8
store i64 %v1, i64* bitcast (i32** @out to i64*), align 8
%inc = add nsw i32 %i.0, 1
%cmp = icmp slt i32 %inc, 100
br i1 %cmp, label %do.body, label %do.end
do.end: ; preds = %do.body
ret void
}
define void @unroll_full() #0 {
entry:
br label %do.body
do.body: ; preds = %entry
%i.0 = phi i32 [ 0, %entry ], [ %inc, %do.body ]
%v1 = load i64, i64* bitcast (i32** @in to i64*), align 8
store i64 %v1, i64* bitcast (i32** @out to i64*), align 8
%inc = add nsw i32 %i.0, 1
%cmp = icmp slt i32 %inc, 100
br i1 %cmp, label %do.body, label %do.end
do.end: ; preds = %do.body
ret void
}
attributes #0 = { "amdgpu-unroll-threshold"="1000" }