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some random notes.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60624 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2008-12-06 19:28:22 +00:00
parent 4a3136444e
commit 78a7e7c797
1 changed files with 97 additions and 0 deletions
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97
lib/Target/README.txt
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@ -1272,4 +1272,101 @@ the function, e.g. by:
later.
//===---------------------------------------------------------------------===//
Store sinking: This code:
void f (int n, int *cond, int *res) {
int i;
*res = 0;
for (i = 0; i < n; i++)
if (*cond)
*res ^= 234; /* (*) */
}
On this function GVN hoists the fully redundant value of *res, but nothing
moves the store out. This gives us this code:
bb: ; preds = %bb2, %entry
%.rle = phi i32 [ 0, %entry ], [ %.rle6, %bb2 ]
%i.05 = phi i32 [ 0, %entry ], [ %indvar.next, %bb2 ]
%1 = load i32* %cond, align 4
%2 = icmp eq i32 %1, 0
br i1 %2, label %bb2, label %bb1
bb1: ; preds = %bb
%3 = xor i32 %.rle, 234
store i32 %3, i32* %res, align 4
br label %bb2
bb2: ; preds = %bb, %bb1
%.rle6 = phi i32 [ %3, %bb1 ], [ %.rle, %bb ]
%indvar.next = add i32 %i.05, 1
%exitcond = icmp eq i32 %indvar.next, %n
br i1 %exitcond, label %return, label %bb
DSE should sink partially dead stores to get the store out of the loop.
//===---------------------------------------------------------------------===//
Scalar PRE hoists the mul in the common block up to the else:
int test (int a, int b, int c, int g) {
int d, e;
if (a)
d = b * c;
else
d = b - c;
e = b * c + g;
return d + e;
}
It would be better to do the mul once to reduce codesize above the if.
This is GCC PR38204.
//===---------------------------------------------------------------------===//
GCC PR37810 is an interesting case where we should sink load/store reload
into the if block and outside the loop, so we don't reload/store it on the
non-call path.
for () {
*P += 1;
if ()
call();
else
...
->
tmp = *P
for () {
tmp += 1;
if () {
*P = tmp;
call();
tmp = *P;
} else ...
}
*P = tmp;
//===---------------------------------------------------------------------===//
GCC PR37166: Sinking of loads prevents SROA'ing the "g" struct on the stack
leading to excess stack traffic. This could be handled by GVN with some crazy
symbolic phi translation. The code we get looks like (g is on the stack):
bb2: ; preds = %bb1
..
%9 = getelementptr %struct.f* %g, i32 0, i32 0
store i32 %8, i32* %9, align bel %bb3
bb3: ; preds = %bb1, %bb2, %bb
%c_addr.0 = phi %struct.f* [ %g, %bb2 ], [ %c, %bb ], [ %c, %bb1 ]
%b_addr.0 = phi %struct.f* [ %b, %bb2 ], [ %g, %bb ], [ %b, %bb1 ]
%10 = getelementptr %struct.f* %c_addr.0, i32 0, i32 0
%11 = load i32* %10, align 4
%11 is fully redundant, an in BB2 it should have the value %8.
//===---------------------------------------------------------------------===//