Two Build Errors with BUILD_CONFIG="bootstrap-O1"

[Xi Ruoyao]

On Tue, 2025-02-04 at 15:03 +0000, JohnyTheCarrot via Gcc-help wrote:
> Hi all,
> 
> I have built GCC with BUILD_CONFIG="bootstrap-O1"
> with the intention of contributing, hence O1.
> I ran across two separate build errors, however.
> Online, I have found mentions of others running
> into similar errors on O1.
> 
> I have made a local patch to fix both build
> errors and have since been able to successfully build GCC.
> I was wondering if I did something wrong, perhaps?
> Does no-one use O1 to build when working on GCC? What do
> contributors' build settings usually look like?
> If others did, surely this would have been noticed, no?

I never used it and I don't think it's very useful.

> Both errors are related to uninitialized variables.
> One at https://github.com/gcc-mirror/gcc/blob/master/gcc/config/i386/i386-options.cc#L3085
> and one at https://github.com/gcc-mirror/gcc/blob/master/gcc/tree-tailcall.cc#L472
> 
> If welcome, I can submit my local patches for review.

To me it may be better to just do something like

diff --git a/config/bootstrap-O1.mk b/config/bootstrap-O1.mk
index 031645a1251..8f47739a04b 100644
--- a/config/bootstrap-O1.mk
+++ b/config/bootstrap-O1.mk
@@ -1 +1 @@
-BOOT_CFLAGS := -O1 $(filter-out -O%, $(BOOT_CFLAGS))
+BOOT_CFLAGS := -O1 $(filter-out -O%, $(BOOT_CFLAGS)) -Wno-maybe-uninitialized


instead of "fixing" the "uninitialized" variables here and there.  There
are no such warnings at -O2, indicating the variables are never really
used uninitialized but at -O1 the optimization is just not enough to
prove that.

Also unnecessarily "initializing" a variable is bad: in case -O2
regresses to fail to recognize a variable is actually always
initialized, we immediately get a bootstrap failure and we notice the
regression.  But if the variable is unnecessarily initialized, when -O2
regresses we wouldn't notice it.

And you always have --disable-werror, generally (i.e. for GCC or other
packages) when you alters the optimization level you likely need to use
it.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University