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Don't PRE compares. 

CodeGenPrepare sinks compare instructions down to their uses to prevent
live flags and predicate registers across basic blocks.

PRE of a compare instruction prevents that, forcing the i1 compare
result into a general purpose register.  That is usually more expensive
than the redundant compare PRE was trying to eliminate in the first
place.

llvm-svn: 153657
This commit is contained in:
Jakob Stoklund Olesen 2012-03-29 17:22:39 +00:00
parent 3e2320c8be
commit 4e55044ff5
2 changed files with 76 additions and 1 deletions
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9
llvm/lib/Transforms/Scalar/GVN.cpp
View File

@ -2328,7 +2328,14 @@ bool GVN::performPRE(Function &F) {
CurInst->mayReadFromMemory() || CurInst->mayHaveSideEffects() ||
isa<DbgInfoIntrinsic>(CurInst))
continue;
// Don't do PRE on compares. The PHI would prevent CodeGenPrepare from
// sinking the compare again, and it would force the code generator to
// move the i1 from processor flags or predicate registers into a general
// purpose register.
if (isa<CmpInst>(CurInst))
continue;
// We don't currently value number ANY inline asm calls.
if (CallInst *CallI = dyn_cast<CallInst>(CurInst))
if (CallI->isInlineAsm())

68
llvm/test/Transforms/GVN/pre-compare.ll Normal file
View File

@ -0,0 +1,68 @@
; RUN: opt -gvn -S < %s | FileCheck %s
; C source:
;
; void f(int x) {
; if (x != 1)
; puts (x == 2 ? "a" : "b");
; for (;;) {
; puts("step 1");
; if (x == 2)
; continue;
; printf("step 2: %d\n", x);
; }
; }
;
; If we PRE %cmp3, CodeGenPrepare won't be able to sink the compare down to its
; uses, and we are forced to keep both %x and %cmp3 in registers in the loop.
;
; It is just as cheap to recompute the icmp against %x as it is to compare a
; GPR against 0. On x86-64, the br i1 %cmp3 becomes:
;
; testb %r12b, %r12b
; jne LBB0_3
;
; The sunk icmp is:
;
; cmpl $2, %ebx
; je LBB0_3
;
; This is just as good, and it doesn't require a separate register.
;
; CHECK-NOT: phi i1
@.str = private unnamed_addr constant [2 x i8] c"a\00", align 1
@.str1 = private unnamed_addr constant [2 x i8] c"b\00", align 1
@.str2 = private unnamed_addr constant [7 x i8] c"step 1\00", align 1
@.str3 = private unnamed_addr constant [12 x i8] c"step 2: %d\0A\00", align 1
define void @f(i32 %x) noreturn nounwind uwtable ssp {
entry:
%cmp = icmp eq i32 %x, 1
br i1 %cmp, label %for.cond.preheader, label %if.then
if.then: ; preds = %entry
%cmp1 = icmp eq i32 %x, 2
%cond = select i1 %cmp1, i8* getelementptr inbounds ([2 x i8]* @.str, i64 0, i64 0), i8* getelementptr inbounds ([2 x i8]* @.str1, i64 0, i64 0)
%call = tail call i32 @puts(i8* %cond) nounwind
br label %for.cond.preheader
for.cond.preheader: ; preds = %entry, %if.then
%cmp3 = icmp eq i32 %x, 2
br label %for.cond
for.cond: ; preds = %for.cond.backedge, %for.cond.preheader
%call2 = tail call i32 @puts(i8* getelementptr inbounds ([7 x i8]* @.str2, i64 0, i64 0)) nounwind
br i1 %cmp3, label %for.cond.backedge, label %if.end5
if.end5: ; preds = %for.cond
%call6 = tail call i32 (i8*, ...)* @printf(i8* getelementptr inbounds ([12 x i8]* @.str3, i64 0, i64 0), i32 %x) nounwind
br label %for.cond.backedge
for.cond.backedge: ; preds = %if.end5, %for.cond
br label %for.cond
}
declare i32 @puts(i8* nocapture) nounwind
declare i32 @printf(i8* nocapture, ...) nounwind