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Differential D64387

CodeGen: Do not memset if following stores will overwrite entire variable
AbandonedPublic

Authored by vitalybuka on Jul 8 2019, 10:56 PM.

Details

Reviewers
eugenis
pcc
jfb
Summary

In some cases llvm::getMostFrequentByte can't analyze a
constant so we write zeros and then override the rest with stores.
These zeros are not needed and we can keep only stores.

Diff Detail

Revision Contents

PathSize
clang/
lib/
CodeGen/
13 lines
test/
CodeGen/
2 lines

Diff 208600

clang/lib/CodeGen/CGDecl.cpp

Show First 20 Lines • Show All 967 Lines • ▼ Show 20 Lines if (0) {
} }
} }
// TODO: Tweak these constants or use "density". // TODO: Tweak these constants or use "density".
uint64_t SizeLimit = 32; uint64_t SizeLimit = 32;
if (GlobalSize <= SizeLimit) if (GlobalSize <= SizeLimit)
return nullptr; return nullptr;
if (!MFB.Value) {
// We don't know most frequent byte, we may still completely initialize it // We don't know most frequent byte, we may still completely initialize it
// with small number of stores. // with small number of stores.
// TODO: Use llvm::UndefValue::get. If we get here and later we decide if (!MFB.Value)
// that number of stores is small enough to use memset+store then store are MFB.Value = llvm::UndefValue::get(CGM.Int8Ty);
// going to replace entire bzeroed range. With UndefValue we can skip that
// bogus memset. I will do that as a separate patch.
MFB.Value = llvm::ConstantInt::get(CGM.Int8Ty, 0);
}
unsigned StoreBudget = 6; unsigned StoreBudget = 6;
return canEmitStoresForInitAfterMemset(CGM, Init, MFB.Value, StoreBudget) return canEmitStoresForInitAfterMemset(CGM, Init, MFB.Value, StoreBudget)
? MFB.Value ? MFB.Value
: nullptr; : nullptr;
} }
/// Decide whether we want to split a constant structure or array store into a /// Decide whether we want to split a constant structure or array store into a
▲ Show 20 Lines • Show All 1,544 Lines • Show Last 20 Lines

clang/test/CodeGen/init-memset.c

Show First 20 Lines • Show All 43 Lines • ▼ Show 20 Linesvoid test_most_a() {
// CHECK: call void @llvm.memset.{{.*}} // CHECK: call void @llvm.memset.{{.*}}
// CHECK: store i8 122 // CHECK: store i8 122
use(a); use(a);
} }
void test_pointers() { void test_pointers() {
// CHECK-LABEL: define void @test_pointers() // CHECK-LABEL: define void @test_pointers()
void *a[] = {&use, &use, &use, &use, &use, &use}; void *a[] = {&use, &use, &use, &use, &use, &use};
// CHECK: call void @llvm.memset.{{.*}} // CHECK-NOT: @llvm.memset.
// CHECK: store i8* // CHECK: store i8*
// CHECK: store i8* // CHECK: store i8*
// CHECK: store i8* // CHECK: store i8*
// CHECK: store i8* // CHECK: store i8*
// CHECK: store i8* // CHECK: store i8*
// CHECK: store i8* // CHECK: store i8*
use(a); use(a);
} }