Add comments about the differences between Clang and rustc target CPUs

Keep track of differences between Clang and rust choices about default
CPUs for 32-bit x86 targets.

src/librustc_back/target/i686_apple_darwin.rs

@@ -12,7 +12,12 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::apple_base::opts();
+
+    // Use yonah as default CPU to enable SSE[2] instructions.
+    // Clang defaults to i686 for this target, but defaulting to yonah
+    // is consistent with what Clang does for i386-apple-darwin.
     base.cpu = "yonah".to_string();
+
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/i686_linux_android.rs

@@ -12,6 +12,11 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::android_base::opts();
+
+    // Use pentium4 as default CPU to enable SSE[2] instructions.
+    // Clang defaults to i686 and enables SSSE3 for this target, but
+    // defaulting to pentium4 is consistent with linux and windows
+    // targets.
     base.cpu = "pentium4".to_string();
 
     Target {

src/librustc_back/target/i686_pc_windows_gnu.rs

@@ -12,6 +12,9 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::windows_base::opts();
+
+    // Use pentium4 as default CPU to enable SSE[2] instructions.
+    // Clang uses the same default.
     base.cpu = "pentium4".to_string();
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address

src/librustc_back/target/i686_pc_windows_msvc.rs

@@ -12,6 +12,9 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::windows_msvc_base::opts();
+
+    // Use pentium4 as default CPU to enable SSE[2] instructions.
+    // Clang uses the same default.
     base.cpu = "pentium4".to_string();
 
     // Mark all dynamic libraries and executables as compatible with the larger 4GiB address

src/librustc_back/target/i686_unknown_dragonfly.rs

@@ -12,7 +12,11 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::dragonfly_base::opts();
+
+    // Use pentium4 as default CPU to enable SSE[2] instructions.
+    // Clang uses the same default.
     base.cpu = "pentium4".to_string();
+
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/i686_unknown_freebsd.rs

@@ -12,7 +12,12 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::freebsd_base::opts();
+
+    // Use pentium4 as default CPU to enable SSE[2] instructions.
+    // Clang defaults to i486 for this target, but defaulting to
+    // pentium4 is consistent with linux and windows targets.
     base.cpu = "pentium4".to_string();
+
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/i686_unknown_linux_gnu.rs

@@ -12,7 +12,11 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::linux_base::opts();
+
+    // Use pentium4 as default CPU to enable SSE[2] instructions.
+    // Clang uses the same default.
     base.cpu = "pentium4".to_string();
+
     base.pre_link_args.push("-m32".to_string());
 
     Target {

src/librustc_back/target/i686_unknown_linux_musl.rs

@@ -14,7 +14,11 @@ use target::Target;
 
 pub fn target() -> Target {
     let mut base = super::linux_base::opts();
+
+    // Use pentium4 as default CPU to enable SSE[2] instructions.
+    // Clang uses the same default.
     base.cpu = "pentium4".to_string();
+
     base.pre_link_args.push("-m32".to_string());
     base.pre_link_args.push("-Wl,-melf_i386".to_string());