From 9fcb87c352b398aa650310160346c8d9bfcdcc45 Mon Sep 17 00:00:00 2001
From: Jorropo <jorropo.pgm@gmail.com>
Date: Tue, 12 Aug 2025 12:49:13 +0200
Subject: cmd/compile: teach prove about len's & cap's max based on the element
 size

Change-Id: I88056fada1ff488c199fce54cf737dbdd091214d
Reviewed-on: https://go-review.googlesource.com/c/go/+/695095
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
---
 src/cmd/compile/internal/ssa/prove.go | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

(limited to 'src')

diff --git a/src/cmd/compile/internal/ssa/prove.go b/src/cmd/compile/internal/ssa/prove.go
index b9b5d3386d..309229b4d7 100644
--- a/src/cmd/compile/internal/ssa/prove.go
+++ b/src/cmd/compile/internal/ssa/prove.go
@@ -1619,7 +1619,16 @@ func initLimit(v *Value) limit {
 		lim = lim.unsignedMax(1)
 
 	// length operations
-	case OpStringLen, OpSliceLen, OpSliceCap:
+	case OpSliceLen, OpSliceCap:
+		f := v.Block.Func
+		elemSize := uint64(v.Args[0].Type.Elem().Size())
+		if elemSize > 0 {
+			heapSize := uint64(1)<<(uint64(f.Config.PtrSize)*8) - 1
+			maximumElementsFittingInHeap := heapSize / elemSize
+			lim = lim.unsignedMax(maximumElementsFittingInHeap)
+		}
+		fallthrough
+	case OpStringLen:
 		lim = lim.signedMin(0)
 	}
 
-- 
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