[llvm] r333702 - [LoopIdiomRecognize] Only convert loops to ctlz if we can prove that the input is non-negative.
Craig Topper via llvm-commits
llvm-commits at lists.llvm.org
Thu May 31 15:16:55 PDT 2018
Author: ctopper
Date: Thu May 31 15:16:55 2018
New Revision: 333702
URL: http://llvm.org/viewvc/llvm-project?rev=333702&view=rev
Log:
[LoopIdiomRecognize] Only convert loops to ctlz if we can prove that the input is non-negative.
Summary:
Loop idiom recognize tries to convert loops like
```
int foo(int x) {
int cnt = 0;
while (x) {
x >>= 1;
++cnt;
}
return cnt;
}
```
into calls to ctlz, but if x is initially negative this loop should be infinite.
It happens that the cases that motivated this change have an absolute value of x before the loop. So this patch restricts the transform to cases where we know x is positive. Note: We are relying on the absolute value of INT_MIN to be undefined so we can assume that the result is always positive.
Fixes PR37479
Reviewers: spatel, hfinkel, efriedma, javed.absar
Reviewed By: efriedma
Subscribers: dmgreen, llvm-commits
Differential Revision: https://reviews.llvm.org/D47348
Modified:
llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
llvm/trunk/test/Transforms/LoopIdiom/ARM/ctlz.ll
llvm/trunk/test/Transforms/LoopIdiom/X86/ctlz.ll
Modified: llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp?rev=333702&r1=333701&r2=333702&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopIdiomRecognize.cpp Thu May 31 15:16:55 2018
@@ -1316,6 +1316,7 @@ static bool detectCTLZIdiom(Loop *CurLoo
return false;
// step 2: detect instructions corresponding to "x.next = x >> 1"
+ // TODO: Support loops that use LShr.
if (!DefX || DefX->getOpcode() != Instruction::AShr)
return false;
ConstantInt *Shft = dyn_cast<ConstantInt>(DefX->getOperand(1));
@@ -1397,6 +1398,13 @@ bool LoopIdiomRecognize::recognizeAndIns
// parent function RunOnLoop.
BasicBlock *PH = CurLoop->getLoopPreheader();
Value *InitX = PhiX->getIncomingValueForBlock(PH);
+
+ // Make sure the initial value can't be negative otherwise the ashr in the
+ // loop might never reach zero which would make the loop infinite.
+ // TODO: Support loops that use lshr and wouldn't need this check.
+ if (!isKnownNonNegative(InitX, *DL))
+ return false;
+
// If we check X != 0 before entering the loop we don't need a zero
// check in CTLZ intrinsic, but only if Cnt Phi is not used outside of the
// loop (if it is used we count CTLZ(X >> 1)).
Modified: llvm/trunk/test/Transforms/LoopIdiom/ARM/ctlz.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopIdiom/ARM/ctlz.ll?rev=333702&r1=333701&r2=333702&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopIdiom/ARM/ctlz.ll (original)
+++ llvm/trunk/test/Transforms/LoopIdiom/ARM/ctlz.ll Thu May 31 15:16:55 2018
@@ -7,6 +7,7 @@
;
; int ctlz_and_other(int n, char *a)
; {
+; n = n >= 0 ? n : -n;
; int i = 0, n0 = n;
; while(n >>= 1) {
; a[i] = (n0 & (1 << i)) ? 1 : 0;
@@ -30,7 +31,10 @@
; Function Attrs: norecurse nounwind uwtable
define i32 @ctlz_and_other(i32 %n, i8* nocapture %a) {
entry:
- %shr8 = ashr i32 %n, 1
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
+ %shr8 = lshr i32 %abs_n, 1
%tobool9 = icmp eq i32 %shr8, 0
br i1 %tobool9, label %while.end, label %while.body.preheader
@@ -42,7 +46,7 @@ while.body:
%shr11 = phi i32 [ %shr, %while.body ], [ %shr8, %while.body.preheader ]
%0 = trunc i64 %indvars.iv to i32
%shl = shl i32 1, %0
- %and = and i32 %shl, %n
+ %and = and i32 %shl, %abs_n
%tobool1 = icmp ne i32 %and, 0
%conv = zext i1 %tobool1 to i8
%arrayidx = getelementptr inbounds i8, i8* %a, i64 %indvars.iv
@@ -67,6 +71,7 @@ while.end:
;
; int ctlz_zero_check(int n)
; {
+; n = n >= 0 ? n : -n;
; int i = 0;
; while(n) {
; n >>= 1;
@@ -76,7 +81,7 @@ while.end:
; }
;
; ALL: entry
-; ALL: %0 = call i32 @llvm.ctlz.i32(i32 %n, i1 true)
+; ALL: %0 = call i32 @llvm.ctlz.i32(i32 %abs_n, i1 true)
; ALL-NEXT: %1 = sub i32 32, %0
; ALL: %inc.lcssa = phi i32 [ %1, %while.body ]
; ALL: %i.0.lcssa = phi i32 [ 0, %entry ], [ %inc.lcssa, %while.end.loopexit ]
@@ -85,7 +90,10 @@ while.end:
; Function Attrs: norecurse nounwind readnone uwtable
define i32 @ctlz_zero_check(i32 %n) {
entry:
- %tobool4 = icmp eq i32 %n, 0
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
+ %tobool4 = icmp eq i32 %abs_n, 0
br i1 %tobool4, label %while.end, label %while.body.preheader
while.body.preheader: ; preds = %entry
@@ -93,7 +101,7 @@ while.body.preheader:
while.body: ; preds = %while.body.preheader, %while.body
%i.06 = phi i32 [ %inc, %while.body ], [ 0, %while.body.preheader ]
- %n.addr.05 = phi i32 [ %shr, %while.body ], [ %n, %while.body.preheader ]
+ %n.addr.05 = phi i32 [ %shr, %while.body ], [ %abs_n, %while.body.preheader ]
%shr = ashr i32 %n.addr.05, 1
%inc = add nsw i32 %i.06, 1
%tobool = icmp eq i32 %shr, 0
@@ -113,6 +121,7 @@ while.end:
;
; int ctlz(int n)
; {
+; n = n >= 0 ? n : -n;
; int i = 0;
; while(n >>= 1) {
; i++;
@@ -121,7 +130,7 @@ while.end:
; }
;
; ALL: entry
-; ALL: %0 = ashr i32 %n, 1
+; ALL: %0 = ashr i32 %abs_n, 1
; ALL-NEXT: %1 = call i32 @llvm.ctlz.i32(i32 %0, i1 false)
; ALL-NEXT: %2 = sub i32 32, %1
; ALL-NEXT: %3 = add i32 %2, 1
@@ -131,10 +140,13 @@ while.end:
; Function Attrs: norecurse nounwind readnone uwtable
define i32 @ctlz(i32 %n) {
entry:
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
br label %while.cond
while.cond: ; preds = %while.cond, %entry
- %n.addr.0 = phi i32 [ %n, %entry ], [ %shr, %while.cond ]
+ %n.addr.0 = phi i32 [ %abs_n, %entry ], [ %shr, %while.cond ]
%i.0 = phi i32 [ 0, %entry ], [ %inc, %while.cond ]
%shr = ashr i32 %n.addr.0, 1
%tobool = icmp eq i32 %shr, 0
@@ -151,6 +163,7 @@ while.end:
;
; int ctlz_add(int n, int i0)
; {
+; n = n >= 0 ? n : -n;
; int i = i0;
; while(n >>= 1) {
; i++;
@@ -159,7 +172,7 @@ while.end:
; }
;
; ALL: entry
-; ALL: %0 = ashr i32 %n, 1
+; ALL: %0 = ashr i32 %abs_n, 1
; ALL-NEXT: %1 = call i32 @llvm.ctlz.i32(i32 %0, i1 false)
; ALL-NEXT: %2 = sub i32 32, %1
; ALL-NEXT: %3 = add i32 %2, 1
@@ -170,10 +183,13 @@ while.end:
; Function Attrs: norecurse nounwind readnone uwtable
define i32 @ctlz_add(i32 %n, i32 %i0) {
entry:
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
br label %while.cond
while.cond: ; preds = %while.cond, %entry
- %n.addr.0 = phi i32 [ %n, %entry ], [ %shr, %while.cond ]
+ %n.addr.0 = phi i32 [ %abs_n, %entry ], [ %shr, %while.cond ]
%i.0 = phi i32 [ %i0, %entry ], [ %inc, %while.cond ]
%shr = ashr i32 %n.addr.0, 1
%tobool = icmp eq i32 %shr, 0
Modified: llvm/trunk/test/Transforms/LoopIdiom/X86/ctlz.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopIdiom/X86/ctlz.ll?rev=333702&r1=333701&r2=333702&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopIdiom/X86/ctlz.ll (original)
+++ llvm/trunk/test/Transforms/LoopIdiom/X86/ctlz.ll Thu May 31 15:16:55 2018
@@ -7,6 +7,7 @@
;
; int ctlz_and_other(int n, char *a)
; {
+; n = n >= 0 ? n : -n;
; int i = 0, n0 = n;
; while(n >>= 1) {
; a[i] = (n0 & (1 << i)) ? 1 : 0;
@@ -30,7 +31,10 @@
; Function Attrs: norecurse nounwind uwtable
define i32 @ctlz_and_other(i32 %n, i8* nocapture %a) {
entry:
- %shr8 = ashr i32 %n, 1
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
+ %shr8 = lshr i32 %abs_n, 1
%tobool9 = icmp eq i32 %shr8, 0
br i1 %tobool9, label %while.end, label %while.body.preheader
@@ -42,7 +46,7 @@ while.body:
%shr11 = phi i32 [ %shr, %while.body ], [ %shr8, %while.body.preheader ]
%0 = trunc i64 %indvars.iv to i32
%shl = shl i32 1, %0
- %and = and i32 %shl, %n
+ %and = and i32 %shl, %abs_n
%tobool1 = icmp ne i32 %and, 0
%conv = zext i1 %tobool1 to i8
%arrayidx = getelementptr inbounds i8, i8* %a, i64 %indvars.iv
@@ -67,6 +71,7 @@ while.end:
;
; int ctlz_zero_check(int n)
; {
+; n = n >= 0 ? n : -n;
; int i = 0;
; while(n) {
; n >>= 1;
@@ -76,7 +81,7 @@ while.end:
; }
;
; ALL: entry
-; ALL: %0 = call i32 @llvm.ctlz.i32(i32 %n, i1 true)
+; ALL: %0 = call i32 @llvm.ctlz.i32(i32 %abs_n, i1 true)
; ALL-NEXT: %1 = sub i32 32, %0
; ALL: %inc.lcssa = phi i32 [ %1, %while.body ]
; ALL: %i.0.lcssa = phi i32 [ 0, %entry ], [ %inc.lcssa, %while.end.loopexit ]
@@ -85,7 +90,10 @@ while.end:
; Function Attrs: norecurse nounwind readnone uwtable
define i32 @ctlz_zero_check(i32 %n) {
entry:
- %tobool4 = icmp eq i32 %n, 0
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
+ %tobool4 = icmp eq i32 %abs_n, 0
br i1 %tobool4, label %while.end, label %while.body.preheader
while.body.preheader: ; preds = %entry
@@ -93,7 +101,7 @@ while.body.preheader:
while.body: ; preds = %while.body.preheader, %while.body
%i.06 = phi i32 [ %inc, %while.body ], [ 0, %while.body.preheader ]
- %n.addr.05 = phi i32 [ %shr, %while.body ], [ %n, %while.body.preheader ]
+ %n.addr.05 = phi i32 [ %shr, %while.body ], [ %abs_n, %while.body.preheader ]
%shr = ashr i32 %n.addr.05, 1
%inc = add nsw i32 %i.06, 1
%tobool = icmp eq i32 %shr, 0
@@ -113,6 +121,7 @@ while.end:
;
; int ctlz(int n)
; {
+; n = n >= 0 ? n : -n;
; int i = 0;
; while(n >>= 1) {
; i++;
@@ -121,7 +130,7 @@ while.end:
; }
;
; ALL: entry
-; ALL: %0 = ashr i32 %n, 1
+; ALL: %0 = ashr i32 %abs_n, 1
; ALL-NEXT: %1 = call i32 @llvm.ctlz.i32(i32 %0, i1 false)
; ALL-NEXT: %2 = sub i32 32, %1
; ALL-NEXT: %3 = add i32 %2, 1
@@ -131,10 +140,13 @@ while.end:
; Function Attrs: norecurse nounwind readnone uwtable
define i32 @ctlz(i32 %n) {
entry:
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
br label %while.cond
while.cond: ; preds = %while.cond, %entry
- %n.addr.0 = phi i32 [ %n, %entry ], [ %shr, %while.cond ]
+ %n.addr.0 = phi i32 [ %abs_n, %entry ], [ %shr, %while.cond ]
%i.0 = phi i32 [ 0, %entry ], [ %inc, %while.cond ]
%shr = ashr i32 %n.addr.0, 1
%tobool = icmp eq i32 %shr, 0
@@ -151,6 +163,7 @@ while.end:
;
; int ctlz_add(int n, int i0)
; {
+; n = n >= 0 ? n : -n;
; int i = i0;
; while(n >>= 1) {
; i++;
@@ -159,7 +172,7 @@ while.end:
; }
;
; ALL: entry
-; ALL: %0 = ashr i32 %n, 1
+; ALL: %0 = ashr i32 %abs_n, 1
; ALL-NEXT: %1 = call i32 @llvm.ctlz.i32(i32 %0, i1 false)
; ALL-NEXT: %2 = sub i32 32, %1
; ALL-NEXT: %3 = add i32 %2, 1
@@ -170,10 +183,13 @@ while.end:
; Function Attrs: norecurse nounwind readnone uwtable
define i32 @ctlz_add(i32 %n, i32 %i0) {
entry:
+ %c = icmp sgt i32 %n, 0
+ %negn = sub nsw i32 0, %n
+ %abs_n = select i1 %c, i32 %n, i32 %negn
br label %while.cond
while.cond: ; preds = %while.cond, %entry
- %n.addr.0 = phi i32 [ %n, %entry ], [ %shr, %while.cond ]
+ %n.addr.0 = phi i32 [ %abs_n, %entry ], [ %shr, %while.cond ]
%i.0 = phi i32 [ %i0, %entry ], [ %inc, %while.cond ]
%shr = ashr i32 %n.addr.0, 1
%tobool = icmp eq i32 %shr, 0
@@ -189,7 +205,6 @@ while.end:
; all ones and continue doing so. This prevents the loop from terminating. If
; we convert this to a countable loop using ctlz that loop will only run 32
; times. This is different than the infinite number of times of the original.
-; FIXME: Don't transform this loop.
define i32 @foo(i32 %x) {
; LZCNT-LABEL: @foo(
; LZCNT-NEXT: entry:
@@ -197,21 +212,17 @@ define i32 @foo(i32 %x) {
; LZCNT-NEXT: [[TOBOOL4:%.*]] = icmp eq i32 [[X:%.*]], 0
; LZCNT-NEXT: br i1 [[TOBOOL4]], label [[WHILE_END:%.*]], label [[WHILE_BODY_LR_PH:%.*]]
; LZCNT: while.body.lr.ph:
-; LZCNT-NEXT: [[TMP0:%.*]] = call i32 @llvm.ctlz.i32(i32 [[X]], i1 true)
-; LZCNT-NEXT: [[TMP1:%.*]] = sub i32 32, [[TMP0]]
; LZCNT-NEXT: br label [[WHILE_BODY:%.*]]
; LZCNT: while.body:
-; LZCNT-NEXT: [[TCPHI:%.*]] = phi i32 [ [[TMP1]], [[WHILE_BODY_LR_PH]] ], [ [[TCDEC:%.*]], [[WHILE_BODY]] ]
; LZCNT-NEXT: [[CNT_06:%.*]] = phi i32 [ 0, [[WHILE_BODY_LR_PH]] ], [ [[INC:%.*]], [[WHILE_BODY]] ]
; LZCNT-NEXT: [[X_ADDR_05:%.*]] = phi i32 [ [[X]], [[WHILE_BODY_LR_PH]] ], [ [[SHR:%.*]], [[WHILE_BODY]] ]
; LZCNT-NEXT: [[SHR]] = ashr i32 [[X_ADDR_05]], 1
; LZCNT-NEXT: [[INC]] = add i32 [[CNT_06]], 1
; LZCNT-NEXT: store volatile i8 42, i8* [[V]], align 1
-; LZCNT-NEXT: [[TCDEC]] = sub nsw i32 [[TCPHI]], 1
-; LZCNT-NEXT: [[TOBOOL:%.*]] = icmp eq i32 [[TCDEC]], 0
+; LZCNT-NEXT: [[TOBOOL:%.*]] = icmp eq i32 [[SHR]], 0
; LZCNT-NEXT: br i1 [[TOBOOL]], label [[WHILE_COND_WHILE_END_CRIT_EDGE:%.*]], label [[WHILE_BODY]]
; LZCNT: while.cond.while.end_crit_edge:
-; LZCNT-NEXT: [[SPLIT:%.*]] = phi i32 [ [[TMP1]], [[WHILE_BODY]] ]
+; LZCNT-NEXT: [[SPLIT:%.*]] = phi i32 [ [[INC]], [[WHILE_BODY]] ]
; LZCNT-NEXT: br label [[WHILE_END]]
; LZCNT: while.end:
; LZCNT-NEXT: [[CNT_0_LCSSA:%.*]] = phi i32 [ [[SPLIT]], [[WHILE_COND_WHILE_END_CRIT_EDGE]] ], [ 0, [[ENTRY:%.*]] ]
More information about the llvm-commits
mailing list