Add a natural stack alignment field to TargetData, and prevent InstCombine from

promoting allocas to preferred alignments that exceed the natural
alignment. This avoids some potentially expensive dynamic stack realignments.

The natural stack alignment is set in target data strings via the "S<size>"
option. Size is in bits and must be a multiple of 8. The natural stack alignment
defaults to "unspecified" (represented by a zero value), and the "unspecified"
value does not prevent any alignment promotions. Target maintainers that care
about avoiding promotions should explicitly add the "S<size>" option to their
target data strings.

llvm-svn: 141599

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/docs/LangRef.html b/llvm/docs/LangRef.html
index 1392db6e60a..a28d92efa1a 100644
--- a/llvm/docs/LangRef.html
+++ b/llvm/docs/LangRef.html
@@ -1319,6 +1319,12 @@ target datalayout = "<i>layout specification</i>"
       the bits with the least significance have the lowest address
       location.</dd>
 
+  <dt><tt>S<i>size</i></tt></dt>
+  <dd>Specifies the natural alignment of the stack in bits. Alignment promotion
+      of stack variables is limited to the natural stack alignment to avoid
+      dynamic stack realignment. The stack alignment must be a multiple of
+      8-bits, and currently defaults to 128 bits if unspecified.</dd>
+
   <dt><tt>p:<i>size</i>:<i>abi</i>:<i>pref</i></tt></dt>
   <dd>This specifies the <i>size</i> of a pointer and its <i>abi</i> and
       <i>preferred</i> alignments. All sizes are in bits. Specifying