[LLVM][LoopVectorize] Fix SIGFPE crash in getPredBlockCostDivisor

When computing block frequency division in getPredBlockCostDivisor,
the function could perform division by zero if BBFreq equals 0.
This caused a floating point exception crash when processing blocks
with zero block frequency information.

Add a guard check to return 1 when BBFreq is 0, avoiding the undefined
behavior while maintaining correct cost model behavior. A divisor of 1
indicates the block is executed at the same frequency as the header,
which is a safe default when frequency information is unavailable.

Fixes issue #172049

Author: dev-priyanshu15

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -2907,6 +2907,10 @@ uint64_t LoopVectorizationCostModel::getPredBlockCostDivisor(
   uint64_t BBFreq = getBFI().getBlockFreq(BB).getFrequency();
   assert(HeaderFreq >= BBFreq &&
          "Header has smaller block freq than dominated BB?");
+  // Guard against division by zero when BBFreq is 0.
+  // In such cases, return 1 to avoid undefined behavior.
+  if (BBFreq == 0)
+    return 1;
   return std::round((double)HeaderFreq / BBFreq);
 }
 

llvm/test/Transforms/LoopVectorize/crash-sigfpe-zero-freq.ll

@@ -0,0 +1,39 @@
+; Test case for crash with Floating point Exception in loop-vectorize pass
+; This test verifies that the loop vectorizer does not crash with SIGFPE
+; when processing blocks with zero block frequency.
+; See issue #172049
+
+; RUN: opt -passes=loop-vectorize -S %s
+
+; ModuleID = 'reduced.ll'
+source_filename = "reduced.ll"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128-ni:1-p2:32:8:8:32-ni:2"
+target triple = "x86_64-unknown-linux-gnu"
+
+define ptr addrspace(1) @wombat() gc "statepoint-example" {
+bb:
+  br label %bb2
+
+bb1:
+  ret ptr addrspace(1) null
+
+bb2:
+  %phi = phi i64 [ %add, %bb6 ], [ 0, %bb ]
+  br i1 false, label %bb3, label %bb6
+
+bb3:
+  br i1 false, label %bb4, label %bb5, !prof !0
+
+bb4:
+  br label %bb6
+
+bb5:
+  br label %bb6
+
+bb6:
+  %add = add i64 %phi, 1
+  %icmp = icmp eq i64 %phi, 0
+  br i1 %icmp, label %bb2, label %bb1
+}
+
+!0 = !{!"branch_weights", i32 1, i32 0}