[llvm] [SCEV] Use loop guards when checking that RHS >= Start (PR #75039)
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Mon Dec 11 03:25:04 PST 2023
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis
Author: Nikita Popov (nikic)
<details>
<summary>Changes</summary>
Loop guards tend to provide better results when it comes to reasoning about ranges than isLoopEntryGuardedByCond(). See the test change for the motivating case.
I have retained both the loop guard check and the implied cond based check for now, though the latter only seems to impact a single test and only via side effects (nowrap flag calculation) at that.
---
Full diff: https://github.com/llvm/llvm-project/pull/75039.diff
2 Files Affected:
- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+5-1)
- (modified) llvm/test/Analysis/ScalarEvolution/trip-count.ll (+3-3)
``````````diff
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 451ae73dbd601c..580fe112fcd7bd 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -12910,7 +12910,11 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
if (!BECount) {
auto canProveRHSGreaterThanEqualStart = [&]() {
auto CondGE = IsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE;
- if (isLoopEntryGuardedByCond(L, CondGE, OrigRHS, OrigStart))
+ const SCEV *GuardedRHS = applyLoopGuards(OrigRHS, L);
+ const SCEV *GuardedStart = applyLoopGuards(OrigStart, L);
+
+ if (isLoopEntryGuardedByCond(L, CondGE, OrigRHS, OrigStart) ||
+ isKnownPredicate(CondGE, GuardedRHS, GuardedStart))
return true;
// (RHS > Start - 1) implies RHS >= Start.
diff --git a/llvm/test/Analysis/ScalarEvolution/trip-count.ll b/llvm/test/Analysis/ScalarEvolution/trip-count.ll
index 22e49ebdbf4dbf..36b42c62dd3945 100644
--- a/llvm/test/Analysis/ScalarEvolution/trip-count.ll
+++ b/llvm/test/Analysis/ScalarEvolution/trip-count.ll
@@ -127,10 +127,10 @@ leave:
define void @non_zero_from_loop_guard(i16 %n) {
; CHECK-LABEL: 'non_zero_from_loop_guard'
; CHECK-NEXT: Determining loop execution counts for: @non_zero_from_loop_guard
-; CHECK-NEXT: Loop %loop: backedge-taken count is (-1 + (1 umax (%n /u 2)))<nsw>
+; CHECK-NEXT: Loop %loop: backedge-taken count is (-1 + (%n /u 2))<nsw>
; CHECK-NEXT: Loop %loop: constant max backedge-taken count is 32766
-; CHECK-NEXT: Loop %loop: symbolic max backedge-taken count is (-1 + (1 umax (%n /u 2)))<nsw>
-; CHECK-NEXT: Loop %loop: Predicated backedge-taken count is (-1 + (1 umax (%n /u 2)))<nsw>
+; CHECK-NEXT: Loop %loop: symbolic max backedge-taken count is (-1 + (%n /u 2))<nsw>
+; CHECK-NEXT: Loop %loop: Predicated backedge-taken count is (-1 + (%n /u 2))<nsw>
; CHECK-NEXT: Predicates:
; CHECK-NEXT: Loop %loop: Trip multiple is 1
;
``````````
</details>
https://github.com/llvm/llvm-project/pull/75039
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