6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2024-12-15 04:30:12 +00:00
Code Issues Projects Releases Wiki Activity

Enhance memdep to return clobber relation between noalias loads when

Browse Source 
an earlier load could be widened to encompass a later load.  For example,
if we see:

  X = load i8* P, align 4
  Y = load i8* (P+3), align 1

and we have a 32-bit native integer type, we can widen the former load
to i32 which then makes the second load redundant.  GVN can't actually
do anything with this load/load relation yet, so this isn't testable, but 
it is the next step to resolving PR6627, and a fairly general class of 
"merge neighboring loads" missed optimizations.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130250 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2011-04-26 22:42:01 +00:00
parent 9f4b893b84
commit cb5fd743a9
1 changed files with 95 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

98
lib/Analysis/MemoryDependenceAnalysis.cpp
View File

@ -16,6 +16,7 @@
#define DEBUG_TYPE "memdep"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Function.h"
@ -221,6 +222,75 @@ getCallSiteDependencyFrom(CallSite CS, bool isReadOnlyCall,
return MemDepResult::getClobber(ScanIt);
}
/// isLoadLoadClobberIfExtendedToFullWidth - Return true if LI is a load that
/// would fully overlap MemLoc if done as a wider legal integer load.
///
/// MemLocBase, MemLocOffset are lazily computed here the first time the
/// base/offs of memloc is needed.
static bool
isLoadLoadClobberIfExtendedToFullWidth(const AliasAnalysis::Location &MemLoc,
const Value *&MemLocBase,
int64_t &MemLocOffs,
const LoadInst *LI, TargetData *TD) {
// If we have no target data, we can't do this.
if (TD == 0) return false;
// If we haven't already computed the base/offset of MemLoc, do so now.
if (MemLocBase == 0)
MemLocBase = GetPointerBaseWithConstantOffset(MemLoc.Ptr, MemLocOffs, *TD);
// Get the base of this load.
int64_t LIOffs = 0;
const Value *LIBase =
GetPointerBaseWithConstantOffset(LI->getPointerOperand(), LIOffs, *TD);
// If the two pointers are not based on the same pointer, we can't tell that
// they are related.
if (LIBase != MemLocBase) return false;
// Okay, the two values are based on the same pointer, but returned as
// no-alias. This happens when we have things like two byte loads at "P+1"
// and "P+3". Check to see if increasing the size of the "LI" load up to its
// alignment (or the largest native integer type) will allow us to load all
// the bits required by MemLoc.
// If MemLoc is before LI, then no widening of LI will help us out.
if (MemLocOffs < LIOffs) return false;
// Get the alignment of the load in bytes. We assume that it is safe to load
// any legal integer up to this size without a problem. For example, if we're
// looking at an i8 load on x86-32 that is known 1024 byte aligned, we can
// widen it up to an i32 load. If it is known 2-byte aligned, we can widen it
// to i16.
unsigned LoadAlign = LI->getAlignment();
int64_t MemLocEnd = MemLocOffs+MemLoc.Size;
// If no amount of rounding up will let MemLoc fit into LI, then bail out.
if (LIOffs+LoadAlign < MemLocEnd) return false;
// This is the size of the load to try. Start with the next larger power of
// two.
unsigned NewLoadByteSize = LI->getType()->getPrimitiveSizeInBits()/8U;
NewLoadByteSize = NextPowerOf2(NewLoadByteSize);
while (1) {
// If this load size is bigger than our known alignment or would not fit
// into a native integer register, then we fail.
if (NewLoadByteSize > LoadAlign ||
!TD->fitsInLegalInteger(NewLoadByteSize*8))
return false;
// If a load of this width would include all of MemLoc, then we succeed.
if (LIOffs+NewLoadByteSize >= MemLocEnd)
return true;
NewLoadByteSize <<= 1;
}
return false;
}
/// getPointerDependencyFrom - Return the instruction on which a memory
/// location depends. If isLoad is true, this routine ignores may-aliases with
/// read-only operations. If isLoad is false, this routine ignores may-aliases
@ -229,6 +299,9 @@ MemDepResult MemoryDependenceAnalysis::
getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
BasicBlock::iterator ScanIt, BasicBlock *BB) {
const Value *MemLocBase = 0;
int64_t MemLocOffset = 0;
// Walk backwards through the basic block, looking for dependencies.
while (ScanIt != BB->begin()) {
Instruction *Inst = --ScanIt;
@ -242,7 +315,8 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
if (II->getIntrinsicID() == Intrinsic::lifetime_start) {
// FIXME: This only considers queries directly on the invariant-tagged
// pointer, not on query pointers that are indexed off of them. It'd
// be nice to handle that at some point.
// be nice to handle that at some point (the right approach is to use
// GetPointerBaseWithConstantOffset).
if (AA->isMustAlias(AliasAnalysis::Location(II->getArgOperand(1)),
MemLoc))
return MemDepResult::getDef(II);
@ -257,10 +331,24 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
// If we found a pointer, check if it could be the same as our pointer.
AliasAnalysis::AliasResult R = AA->alias(LoadLoc, MemLoc);
if (R == AliasAnalysis::NoAlias)
continue;
if (isLoad) {
if (R == AliasAnalysis::NoAlias) {
// If this is an over-aligned integer load (for example,
// "load i8* %P, align 4") see if it would obviously overlap with the
// queried location if widened to a larger load (e.g. if the queried
// location is 1 byte at P+1). If so, return it as a load/load
// clobber result, allowing the client to decide to widen the load if
// it wants to.
if (const IntegerType *ITy = dyn_cast<IntegerType>(LI->getType()))
if (LI->getAlignment()*8 > ITy->getPrimitiveSizeInBits() &&
isLoadLoadClobberIfExtendedToFullWidth(MemLoc, MemLocBase,
MemLocOffset, LI, TD))
return MemDepResult::getClobber(Inst);
continue;
}
// Must aliased loads are defs of each other.
if (R == AliasAnalysis::MustAlias)
return MemDepResult::getDef(Inst);
@ -275,6 +363,10 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
continue;
}
// Stores don't depend on other no-aliased accesses.
if (R == AliasAnalysis::NoAlias)
continue;
// Stores don't alias loads from read-only memory.
if (AA->pointsToConstantMemory(LoadLoc))
continue;