diff --git a/README.md b/README.md
index 9ddf111869..c69300c030 100644
--- a/README.md
+++ b/README.md
@@ -37,7 +37,7 @@ For other versions, check the git tags of this repository.
 ## Build Instructions
 
 ```
-./build <arch>-<os>-<abi> <mcpu>
+./build <arch>-<os>-<abi> <mcpu> <zig_mode>
 ```
 
 All parameters are required:
@@ -48,6 +48,10 @@ All parameters are required:
  * `<mcpu>`: Replace with a `-mcpu` parameter of Zig. `baseline` is recommended
    and means it will target a generic CPU for the target. `native` means it
    will target the native CPU. See the Zig documentation for more details.
+ * `<zig_mode>`: Replace with any one value from the
+   [std.builtin.OptimizeMode](https://ziglang.org/documentation/master/std/#std.builtin.OptimizeMode)
+   enum. Note this only applies to the final output Zig binary. Intermediary build
+   stages will use ReleaseFast.
 
 Please be aware of the following two CMake environment variables that can
 significantly affect how long it takes to build:
@@ -71,7 +75,7 @@ installed via the Visual Studio installer.
 The script must be run within the `Developer Command Prompt for VS 2019` shell:
 
 ```
-build.bat <arch>-<os>-<abi> <mcpu>
+build.bat [arch>-<os>-<abi] [mcpu] [zig_mode]
 ```
 
 To build for x86 Windows, run the script within the `x86 Native Tools Command Prompt for VS 2019`.
diff --git a/build b/build
index bb9f63c3cc..4bcae96e27 100755
--- a/build
+++ b/build
@@ -5,6 +5,7 @@ set -eu
 
 TARGET="$1" # Example: riscv64-linux-gnu
 MCPU="$2" # Examples: `baseline`, `native`, `generic+v7a`, or `arm1176jzf_s`
+ZIG_OPTIMIZE_MODE="$3"
 
 ROOTDIR="$(pwd)"
 ZIG_VERSION="0.12.0-dev.2058+04ac028a2"
@@ -195,7 +196,7 @@ $ZIG build \
   --search-prefix "$ROOTDIR/out/$TARGET-$MCPU" \
   -Dflat \
   -Dstatic-llvm \
-  -Doptimize=ReleaseFast \
+  -Doptimize="$ZIG_OPTIMIZE_MODE" \
   -Dstrip \
   -Dtarget="$TARGET" \
   -Dcpu="$MCPU" \
diff --git a/build.bat b/build.bat
index d7496f287e..f33d48345e 100644
--- a/build.bat
+++ b/build.bat
@@ -16,6 +16,7 @@ if %ERRORLEVEL% neq 0 (
 
 if "%1" == "" (set "TARGET=x86_64-windows-gnu") ELSE (set TARGET=%~1)
 if "%2" == "" (set "MCPU=native") ELSE (set MCPU=%~2)
+if "%3" == "" (set "ZIG_OPTIMIZE_MODE=ReleaseSafe") ELSE (set ZIG_OPTIMIZE_MODE=%~3)
 if "%VSCMD_ARG_HOST_ARCH%"=="x86" set HOST_TARGET=x86-windows-msvc
 if "%VSCMD_ARG_HOST_ARCH%"=="x64" set HOST_TARGET=x86_64-windows-msvc
 echo Boostrapping targeting %TARGET% (%MCPU%), using %HOST_TARGET% as the host compiler
@@ -266,7 +267,7 @@ cd "%ROOTDIR%\zig"
   --search-prefix "%ROOTDIR%%OUTDIR%\%TARGET%-%MCPU%" ^
   -Dflat ^
   -Dstatic-llvm ^
-  -Doptimize=ReleaseFast ^
+  -Doptimize="%ZIG_OPTIMIZE_MODE%" ^
   -Dstrip ^
   -Dtarget="%TARGET%" ^
   -Dcpu="%MCPU%" ^