[llvm] b061d86 - [SCEV] Compute exit count from overflow check expressed w/ x.with.overflow intrinsics
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Tue Jan 4 09:44:44 PST 2022
Author: Philip Reames
Date: 2022-01-04T09:44:23-08:00
New Revision: b061d86c6930acef1b246874adf2f11e9120894c
URL: https://github.com/llvm/llvm-project/commit/b061d86c6930acef1b246874adf2f11e9120894c
DIFF: https://github.com/llvm/llvm-project/commit/b061d86c6930acef1b246874adf2f11e9120894c.diff
LOG: [SCEV] Compute exit count from overflow check expressed w/ x.with.overflow intrinsics
This ports the logic we generate in instcombine for a single use x.with.overflow check for use in SCEV's analysis. The result is that we can prove trip counts for many checks, and (through existing logic) often discharge them.
Motivation comes from compiling a simple example with -ftrapv.
Differential Revision: https://reviews.llvm.org/D116499
Added:
Modified:
llvm/lib/Analysis/ScalarEvolution.cpp
llvm/test/Analysis/ScalarEvolution/overflow-intrinsics-trip-count.ll
llvm/test/CodeGen/PowerPC/negctr.ll
Removed:
################################################################################
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index d48e81f28af91..513b2c0e5da1d 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8093,6 +8093,29 @@ ScalarEvolution::ExitLimit ScalarEvolution::computeExitLimitFromCondImpl(
return getZero(CI->getType());
}
+ // If we're exiting based on the overflow flag of an x.with.overflow intrinsic
+ // with a constant step, we can form an equivalent icmp predicate and figure
+ // out how many iterations will be taken before we exit.
+ const WithOverflowInst *WO;
+ const APInt *C;
+ if (match(ExitCond, m_ExtractValue<1>(m_WithOverflowInst(WO))) &&
+ match(WO->getRHS(), m_APInt(C))) {
+ ConstantRange NWR =
+ ConstantRange::makeExactNoWrapRegion(WO->getBinaryOp(), *C,
+ WO->getNoWrapKind());
+ CmpInst::Predicate Pred;
+ APInt NewRHSC, Offset;
+ NWR.getEquivalentICmp(Pred, NewRHSC, Offset);
+ if (!ExitIfTrue)
+ Pred = ICmpInst::getInversePredicate(Pred);
+ auto *LHS = getSCEV(WO->getLHS());
+ if (Offset != 0)
+ LHS = getAddExpr(LHS, getConstant(Offset));
+ auto EL = computeExitLimitFromICmp(L, Pred, LHS, getConstant(NewRHSC),
+ ControlsExit, AllowPredicates);
+ if (EL.hasAnyInfo()) return EL;
+ }
+
// If it's not an integer or pointer comparison then compute it the hard way.
return computeExitCountExhaustively(L, ExitCond, ExitIfTrue);
}
diff --git a/llvm/test/Analysis/ScalarEvolution/overflow-intrinsics-trip-count.ll b/llvm/test/Analysis/ScalarEvolution/overflow-intrinsics-trip-count.ll
index 942f312599da2..38372c94e3ea3 100644
--- a/llvm/test/Analysis/ScalarEvolution/overflow-intrinsics-trip-count.ll
+++ b/llvm/test/Analysis/ScalarEvolution/overflow-intrinsics-trip-count.ll
@@ -167,9 +167,11 @@ for.end: ; preds = %for.body, %entry
define void @uadd_symbolic_start(i16 %start) {
; CHECK-LABEL: 'uadd_symbolic_start'
; CHECK-NEXT: Determining loop execution counts for: @uadd_symbolic_start
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable predicated backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (-1 + (-1 * %start))
+; CHECK-NEXT: Loop %for.body: max backedge-taken count is -1
+; CHECK-NEXT: Loop %for.body: Predicated backedge-taken count is (-1 + (-1 * %start))
+; CHECK-NEXT: Predicates:
+; CHECK: Loop %for.body: Trip multiple is 1
;
entry:
br i1 undef, label %for.end, label %for.body.preheader
@@ -191,9 +193,11 @@ for.end: ; preds = %for.body, %entry
define void @sadd_symbolic_start(i16 %start) {
; CHECK-LABEL: 'sadd_symbolic_start'
; CHECK-NEXT: Determining loop execution counts for: @sadd_symbolic_start
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable predicated backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (32767 + (-1 * %start))
+; CHECK-NEXT: Loop %for.body: max backedge-taken count is -1
+; CHECK-NEXT: Loop %for.body: Predicated backedge-taken count is (32767 + (-1 * %start))
+; CHECK-NEXT: Predicates:
+; CHECK: Loop %for.body: Trip multiple is 1
;
entry:
br i1 undef, label %for.end, label %for.body.preheader
@@ -264,9 +268,11 @@ for.end: ; preds = %for.body, %entry
define void @usub_symbolic_start(i16 %start) {
; CHECK-LABEL: 'usub_symbolic_start'
; CHECK-NEXT: Determining loop execution counts for: @usub_symbolic_start
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable predicated backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is %start
+; CHECK-NEXT: Loop %for.body: max backedge-taken count is -1
+; CHECK-NEXT: Loop %for.body: Predicated backedge-taken count is %start
+; CHECK-NEXT: Predicates:
+; CHECK: Loop %for.body: Trip multiple is 1
;
entry:
br i1 undef, label %for.end, label %for.body.preheader
@@ -288,9 +294,11 @@ for.end: ; preds = %for.body, %entry
define void @ssub_symbolic_start(i16 %start) {
; CHECK-LABEL: 'ssub_symbolic_start'
; CHECK-NEXT: Determining loop execution counts for: @ssub_symbolic_start
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable predicated backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (-32768 + %start)
+; CHECK-NEXT: Loop %for.body: max backedge-taken count is -1
+; CHECK-NEXT: Loop %for.body: Predicated backedge-taken count is (-32768 + %start)
+; CHECK-NEXT: Predicates:
+; CHECK: Loop %for.body: Trip multiple is 1
;
entry:
br i1 undef, label %for.end, label %for.body.preheader
@@ -360,11 +368,13 @@ for.end: ; preds = %for.body, %entry
define void @sadd_symbolic_non_latch(i16 %start) {
; CHECK-LABEL: 'sadd_symbolic_non_latch'
; CHECK-NEXT: Determining loop execution counts for: @sadd_symbolic_non_latch
-; CHECK-NEXT: Loop %for.body: <multiple exits> Unpredictable backedge-taken count.
-; CHECK-NEXT: exit count for for.body: ***COULDNOTCOMPUTE***
+; CHECK-NEXT: Loop %for.body: <multiple exits> backedge-taken count is ((230 + (-1 * %start)) umin (32767 + (-1 * %start)))
+; CHECK-NEXT: exit count for for.body: (32767 + (-1 * %start))
; CHECK-NEXT: exit count for for.latch: (230 + (-1 * %start))
; CHECK-NEXT: Loop %for.body: max backedge-taken count is -1
-; CHECK-NEXT: Loop %for.body: Unpredictable predicated backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Predicated backedge-taken count is ((230 + (-1 * %start)) umin (32767 + (-1 * %start)))
+; CHECK-NEXT: Predicates:
+; CHECK: Loop %for.body: Trip multiple is 1
;
entry:
br i1 undef, label %for.end, label %for.body.preheader
diff --git a/llvm/test/CodeGen/PowerPC/negctr.ll b/llvm/test/CodeGen/PowerPC/negctr.ll
index 93c7daed64f61..38664b058b878 100644
--- a/llvm/test/CodeGen/PowerPC/negctr.ll
+++ b/llvm/test/CodeGen/PowerPC/negctr.ll
@@ -34,14 +34,10 @@ for.body: ; preds = %for.body, %entry
%exitcond = icmp eq i64 %indvars.iv.next, 0
br i1 %exitcond, label %for.end, label %for.body
-; FIXME: This should be a hardware loop.
-; cmp is optimized to uadd intrinsic in CGP pass which can not be recognized in
-; later HardwareLoops Pass.
; CHECK: @main1
-; CHECK: li [[REG:[0-9]+]], 1
-; CHECK: addi [[REG2:[0-9]+]], [[REG]], 1
-; CHECK: cmpld
-; CHECK: bge
+; CHECK: li [[REG:[0-9]+]], -1
+; CHECK: mtctr [[REG]]
+; CHECK: bdnz
for.end: ; preds = %for.body, %entry
ret void
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