path: root/configure
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authorRich Felker <>2019-05-11 19:44:21 -0400
committerRich Felker <>2019-05-11 19:44:21 -0400
commitbdb0817599325e6ca6838717dfe18290695a59a2 (patch)
tree9ec908dfc35ac48bef7bf19126147289a3561eb2 /configure
parentc8798ef974d21c338a7d8d874a402978ffc6168e (diff)
improve i386 inline syscall asm on non-broken compilers
we have to avoid using ebx unconditionally in asm constraints for i386, because gcc 3 and 4 and possibly other simplistic compilers (pcc?) implement PIC via making ebx a fixed-use register, and disallow its use for anything else. rather than hard-coding knowledge of which compilers work (at least gcc 5+ and clang), perform a configure test; this should give us the good codegen on any new compilers we don't yet know about. swapping ebx and edx is kept for 1- and 2-arg syscalls because it avoids having any spills/stack-frame at all in small functions. for 6-arg, if ebx is directly usable, the complex shuffling introduced in commit c8798ef974d21c338a7d8d874a402978ffc6168e can be avoided, and ebp can be loaded the same way ebx is in 5-arg syscalls for compilers that don't support direct use of ebx.
Diffstat (limited to 'configure')
1 files changed, 14 insertions, 0 deletions
diff --git a/configure b/configure
index 6310e0b9..2123ddce 100755
--- a/configure
+++ b/configure
@@ -573,6 +573,20 @@ printf "using compiler runtime libraries: %s\n" "$LIBCC"
+if test "$ARCH" = "i386" ; then
+printf "checking whether compiler can use ebx in PIC asm constraints... "
+cat > "$tmpc" <<EOF
+int foo(int x) { __asm__ ( "" : "+b"(x) ); return x; }
+ -c -o /dev/null "$tmpc" >/dev/null 2>&1 ; then
+printf "yes\n"
+printf "no\n"
if test "$ARCH" = "x86_64" ; then
trycppif __ILP32__ "$t" && ARCH=x32