I was having trouble spotting any warnings because the output when compiling is so messy. Fixing that is pretty simple.

Why you should do it.
The warnings (and errors, if there were any) stick out like a sore thumb because they are so much longer. Take a look at the build output before and after following the instructions in this guide.
Before:

C:\projects\3ds\Pasta-CFW\CFW_loader>make
crypto.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/crypto.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/crypto.c -o crypto.o
draw.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/draw.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/draw.c -o draw.o
fs.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/fs.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c -o fs.o
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'DecryptPartition':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:95:9: warning: unused variable 'bytesWritten' [-Wunused-variable]
  size_t bytesWritten;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'InitializeNandCrypto':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:118:8: warning: unused variable 'listSystem' [-Wunused-variable]
  char* listSystem[] = { "4.x", "6.x", "7.x", "9.x" };
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:151:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:164:2: warning: implicit declaration of function 'sdmmc_nand_readsectors' [-Wimplicit-function-declaration]
  sdmmc_nand_readsectors(sector_no + partition / 0x200, numsectors, out);
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:172:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:193:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:205:2: warning: implicit declaration of function 'sdmmc_sdcard_readsectors' [-Wimplicit-function-declaration]
  sdmmc_sdcard_readsectors(sector_no + partition / 0x200, numsectors, out);
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:213:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:226:2: warning: implicit declaration of function 'sdmmc_sdcard_writesectors' [-Wimplicit-function-declaration]
  sdmmc_sdcard_writesectors(sector_no + partition / 0x200, numsectors, out); //Stubbed, i don't wanna risk
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:166:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:187:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:207:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:227:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
hid.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/hid.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/hid.c -o hid.o
i2c.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/i2c.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/i2c.c -o i2c.o
c:/projects/3ds/Pasta-CFW/CFW_loader/source/i2c.c:6:43: warning: no semicolon at end of struct or union
static const struct { u8 bus_id, reg_addr } dev_data[] = {
  ^
main.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/main.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/main.c -o main.o
c:/projects/3ds/Pasta-CFW/CFW_loader/source/main.c: In function 'CFW_NandDumper':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/main.c:299:23: warning: initialization makes pointer from integer without a cast
  unsigned char* buf = 0x21000000;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/main.c:319:5: warning: implicit declaration of function 'sdmmc_nand_readsectors' [-Wimplicit-function-declaration]
  sdmmc_nand_readsectors(count*nsectors, nsectors, buf);
  ^
platform.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/platform.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/platform.c -o platform.o
diskio.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/diskio.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs/diskio.c -o diskio.o
ff.c
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/ff.d -g -Wall -O2 -march=armv5te -mtune=arm946e-s -fomit-frame-pointer -ffast-math -std=c99 -mthumb -mthumb-interwork -I/c/projects/3ds/Pasta-CFW/CFW_loader/include -I/c/projects/3ds/Pasta-CFW/CFW_loader/source -I/c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs  -I/c/projects/3ds/Pasta-CFW/CFW_loader/build -DARM9 -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/fatfs/ff.c -o ff.o
delay.s
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/delay.d -x assembler-with-cpp -g -mthumb -mthumb-interwork -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/delay.s -o delay.o
start.s
arm-none-eabi-gcc -MMD -MP -MF /c/projects/3ds/Pasta-CFW/CFW_loader/build/start.d -x assembler-with-cpp -g -mthumb -mthumb-interwork -c /c/projects/3ds/Pasta-CFW/CFW_loader/source/start.s -o start.o
linking CFW_loader.elf
built ... CFW_loader.bin
cp /c/projects/3ds/Pasta-CFW/CFW_loader/CFW_loader.bin loader.bin
cp /c/projects/3ds/Pasta-CFW/CFW_loader/CFW_loader.elf loader.elf

After:

C:\projects\3ds\Pasta-CFW\CFW_loader>make
crypto.c
draw.c
fs.c
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'DecryptPartition':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:95:9: warning: unused variable 'bytesWritten' [-Wunused-variable]
  size_t bytesWritten;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'InitializeNandCrypto':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:118:8: warning: unused variable 'listSystem' [-Wunused-variable]
  char* listSystem[] = { "4.x", "6.x", "7.x", "9.x" };
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:151:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:164:2: warning: implicit declaration of function 'sdmmc_nand_readsectors' [-Wimplicit-function-declaration]
  sdmmc_nand_readsectors(sector_no + partition / 0x200, numsectors, out);
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:172:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:193:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:205:2: warning: implicit declaration of function 'sdmmc_sdcard_readsectors' [-Wimplicit-function-declaration]
  sdmmc_sdcard_readsectors(sector_no + partition / 0x200, numsectors, out);
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:213:11: warning: assignment from incompatible pointer type
  info.ctr = &myCtr; info.buffer = out; info.size = numsectors * 0x200; info.keyY = NULL;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:226:2: warning: implicit declaration of function 'sdmmc_sdcard_writesectors' [-Wimplicit-function-declaration]
  sdmmc_sdcard_writesectors(sector_no + partition / 0x200, numsectors, out); //Stubbed, i don't wanna risk
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:166:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'nand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:187:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_readsectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:207:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c: In function 'emunand_writesectors':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/fs.c:227:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
hid.c
i2c.c
c:/projects/3ds/Pasta-CFW/CFW_loader/source/i2c.c:6:43: warning: no semicolon at end of struct or union
static const struct { u8 bus_id, reg_addr } dev_data[] = {
  ^
main.c
c:/projects/3ds/Pasta-CFW/CFW_loader/source/main.c: In function 'CFW_NandDumper':
c:/projects/3ds/Pasta-CFW/CFW_loader/source/main.c:299:23: warning: initialization makes pointer from integer without a cast
  unsigned char* buf = 0x21000000;
  ^
c:/projects/3ds/Pasta-CFW/CFW_loader/source/main.c:319:5: warning: implicit declaration of function 'sdmmc_nand_readsectors' [-Wimplicit-function-declaration]
  sdmmc_nand_readsectors(count*nsectors, nsectors, buf);
  ^
platform.c
diskio.c
ff.c
delay.s
start.s
sdmc.s
linking CFW_loader.elf
built ... CFW_loader.bin
cp /c/projects/3ds/Pasta-CFW/CFW_loader/CFW_loader.bin loader.bin
cp /c/projects/3ds/Pasta-CFW/CFW_loader/CFW_loader.elf loader.elf

In addition to that, it's easy to see when there are no more warnings or errors. Here's a build where all the warnings have been fixed (different project):

C:\projects\3ds\3ds_hb_menu>make
app_bubble.bin
battery_charging.bin
battery_full.bin
battery_low.bin
battery_lowest.bin
battery_mid_high.bin
battery_mid_low.bin
bubble.bin
font.bin
installerIcon.bin
logo.bin
regionfree.bin
top_bar.bin
wifi_full.bin
wifi_none.bin
descriptor.cpp
tinyxml2.cpp
background.c
boot.c
error.c
filesystem.c
font.c
font1.c
font2.c
gfx.c
main.c
menu.c
netloader.c
regionfree.c
scanner.c
smdh.c
statusbar.c
text.c
titles.c
water.c
linking 3ds_hb_menu.elf
built ... 3ds_hb_menu.3dsx

A quick scroll through the output is all it takes to see that there are no warnings or errors.

How to do it
Making a clean output could hardly be simpler, and it isn't project-specific. All you need to do is open "C:\devkitPro\devkitARM\base_rules" with a text editor and replace the contents with this. It has no negative effects.

 

[elhobbs]

How is this different than suppressing all warnings with "-w" in CFLAGS?

 

[JoostinOnline]

How is this different than suppressing all warnings with "-w" in CFLAGS?

This is completely the opposite of that. It makes spotting warnings easier. It hides the pointless commands. Read the first line of "Why you should do it".

 

[elhobbs]

This is completely the opposite of that. It makes spotting warnings easier. It hides the pointless commands. Read the first line of "Why you should do it".

Ahh... I see that makes sense. I did not realize the clean output at the end was after the fixing the warnings. I thought it was just hiding them.

 

[mashers]

You can achieve the same result by using "make -s". It inhibits the echoing of the individual compiler commands so you can more easily see any errors and warnings.

 

[JoostinOnline]

You can achieve the same result by using "make -s". It inhibits the echoing of the individual compiler commands so you can more easily see any errors and warnings.

True, but it has two downsides. The first is minor, in that you have to type -s every time. The second is bigger. You will have to modify every batch/bash script by adding a -s command, and the commands used in any tools if you compile through Programmer's Notepad.

This is a simple way that requires no changes to project files.

 

[mashers]

Ahh I see. That is better - thanks!