<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto">Right, strncpy does zeroing, so we need to preserve it.<br><div dir="ltr"><br><blockquote type="cite">Dňa 15. 8. 2020 o 5:19 užívateľ Craig Topper <craig.topper@gmail.com> napísal:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><div dir="ltr">Doesn't strncpy promise that any bytes past the end of the source string are zeroed in the destination?<div><br clear="all"><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature">~Craig</div></div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Fri, Aug 14, 2020 at 7:55 PM Philip Reames via llvm-commits <<a href="mailto:llvm-commits@lists.llvm.org">llvm-commits@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Looking at your example, I'm confused by the need to pad. In the <br>
examples, both the source string length and C are constant. If we know <br>
the source doesn't contain an embedded zero, why not just do a memcpy <br>
for min(Src.length, C)?<br>
<br>
Philip<br>
<br>
On 8/14/20 4:53 PM, Dávid Bolvanský via llvm-commits wrote:<br>
> Author: Dávid Bolvanský<br>
> Date: 2020-08-15T01:53:32+02:00<br>
> New Revision: f62de7c9c71134af060a3a1686e30e69d439e785<br>
><br>
> URL: <a href="https://github.com/llvm/llvm-project/commit/f62de7c9c71134af060a3a1686e30e69d439e785" rel="noreferrer" target="_blank">https://github.com/llvm/llvm-project/commit/f62de7c9c71134af060a3a1686e30e69d439e785</a><br>
> DIFF: <a href="https://github.com/llvm/llvm-project/commit/f62de7c9c71134af060a3a1686e30e69d439e785.diff" rel="noreferrer" target="_blank">https://github.com/llvm/llvm-project/commit/f62de7c9c71134af060a3a1686e30e69d439e785.diff</a><br>
><br>
> LOG: [SLC] Transform strncpy(dst, "text", C) to memcpy(dst, "text\0\0\0", C) for C <= 128 only<br>
><br>
> Transformation creates big strings for big C values, so bail out for C > 128.<br>
><br>
> Reviewed By: efriedma<br>
><br>
> Differential Revision: <a href="https://reviews.llvm.org/D86004" rel="noreferrer" target="_blank">https://reviews.llvm.org/D86004</a><br>
><br>
> Added:<br>
> <br>
><br>
> Modified:<br>
> llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp<br>
> llvm/test/Transforms/InstCombine/strncpy-3.ll<br>
><br>
> Removed:<br>
> <br>
><br>
><br>
> ################################################################################<br>
> diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp<br>
> index 1a7654983e9f..db7078b9283f 100644<br>
> --- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp<br>
> +++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp<br>
> @@ -610,12 +610,16 @@ Value *LibCallSimplifier::optimizeStrNCpy(CallInst *CI, IRBuilderBase &B) {<br>
> <br>
> // strncpy(a, "a", 4) - > memcpy(a, "a\0\0\0", 4)<br>
> if (Len > SrcLen + 1) {<br>
> - StringRef Str;<br>
> - if (!getConstantStringInfo(Src, Str))<br>
> + if (Len <= 128) {<br>
> + StringRef Str;<br>
> + if (!getConstantStringInfo(Src, Str))<br>
> + return nullptr;<br>
> + std::string SrcStr = Str.str();<br>
> + SrcStr.resize(Len, '\0');<br>
> + Src = B.CreateGlobalString(SrcStr, "str");<br>
> + } else {<br>
> return nullptr;<br>
> - std::string SrcStr = Str.str();<br>
> - SrcStr.resize(Len, '\0');<br>
> - Src = B.CreateGlobalString(SrcStr, "str");<br>
> + }<br>
> }<br>
> <br>
> Type *PT = Callee->getFunctionType()->getParamType(0);<br>
><br>
> diff --git a/llvm/test/Transforms/InstCombine/strncpy-3.ll b/llvm/test/Transforms/InstCombine/strncpy-3.ll<br>
> index 744f1e4169e0..28d27a4f3396 100644<br>
> --- a/llvm/test/Transforms/InstCombine/strncpy-3.ll<br>
> +++ b/llvm/test/Transforms/InstCombine/strncpy-3.ll<br>
> @@ -38,3 +38,21 @@ define void @fill_with_zeros3(i8* %dst) {<br>
> tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 4)<br>
> ret void<br>
> }<br>
> +<br>
> +define void @fill_with_zeros4(i8* %dst) {<br>
> +; CHECK-LABEL: @fill_with_zeros4(<br>
> +; CHECK-NEXT: call void @llvm.memcpy.p0i8.p0i8.i64(i8* nonnull align 1 dereferenceable(128) [[DST:%.*]], i8* nonnull align 1 dereferenceable(128) getelementptr inbounds ([129 x i8], [129 x i8]* @str.2, i64 0, i64 0), i64 128, i1 false)<br>
> +; CHECK-NEXT: ret void<br>
> +;<br>
> + tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 128)<br>
> + ret void<br>
> +}<br>
> +<br>
> +define void @no_simplify(i8* %dst) {<br>
> +; CHECK-LABEL: @no_simplify(<br>
> +; CHECK-NEXT: [[TMP1:%.*]] = tail call i8* @strncpy(i8* nonnull dereferenceable(1) [[DST:%.*]], i8* nonnull dereferenceable(5) getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 129)<br>
> +; CHECK-NEXT: ret void<br>
> +;<br>
> + tail call i8* @strncpy(i8* %dst, i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str3, i64 0, i64 0), i64 129)<br>
> + ret void<br>
> +}<br>
><br>
><br>
> <br>
> _______________________________________________<br>
> llvm-commits mailing list<br>
> <a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a><br>
> <a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits" rel="noreferrer" target="_blank">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits</a><br>
_______________________________________________<br>
llvm-commits mailing list<br>
<a href="mailto:llvm-commits@lists.llvm.org" target="_blank">llvm-commits@lists.llvm.org</a><br>
<a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits" rel="noreferrer" target="_blank">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits</a><br>
</blockquote></div>
</div></blockquote></body></html>