qemu-tech: document lazy condition code evaluation in cpu.h

Unlike the other sections, they are pretty specific to a particular CPU.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

6 files changed, 34 insertions, 25 deletions

diff --git a/qemu-tech.texi b/qemu-tech.texi
index 082b62c8e0..75ceea408c 100644
--- a/qemu-tech.texi
+++ b/qemu-tech.texi
@@ -214,7 +214,6 @@ SH4
 @menu
 * QEMU compared to other emulators::
 * Portable dynamic translation::
-* Condition code optimisations::
 * CPU state optimisations::
 * Translation cache::
 * Direct block chaining::
@@ -290,30 +289,6 @@ performances.
 QEMU's dynamic translation backend is called TCG, for "Tiny Code
 Generator". For more information, please take a look at @code{tcg/README}.
 
-@node Condition code optimisations
-@section Condition code optimisations
-
-Lazy evaluation of CPU condition codes (@code{EFLAGS} register on x86)
-is important for CPUs where every instruction sets the condition
-codes. It tends to be less important on conventional RISC systems
-where condition codes are only updated when explicitly requested. On
-Sparc64, costly update of both 32 and 64 bit condition codes can be
-avoided with lazy evaluation.
-
-Instead of computing the condition codes after each x86 instruction,
-QEMU just stores one operand (called @code{CC_SRC}), the result
-(called @code{CC_DST}) and the type of operation (called
-@code{CC_OP}). When the condition codes are needed, the condition
-codes can be calculated using this information. In addition, an
-optimized calculation can be performed for some instruction types like
-conditional branches.
-
-@code{CC_OP} is almost never explicitly set in the generated code
-because it is known at translation time.
-
-The lazy condition code evaluation is used on x86, m68k, cris and
-Sparc. ARM uses a simplified variant for the N and Z flags.
-
 @node CPU state optimisations
 @section CPU state optimisations
 
diff --git a/target-cris/cpu.h b/target-cris/cpu.h
index 7d7fe6eb1c..43d5f9d1da 100644
--- a/target-cris/cpu.h
+++ b/target-cris/cpu.h
@@ -223,6 +223,13 @@ int cpu_cris_signal_handler(int host_signum, void *pinfo,
 void cris_initialize_tcg(void);
 void cris_initialize_crisv10_tcg(void);
 
+/* Instead of computing the condition codes after each CRIS instruction,
+ * QEMU just stores one operand (called CC_SRC), the result
+ * (called CC_DEST) and the type of operation (called CC_OP). When the
+ * condition codes are needed, the condition codes can be calculated
+ * using this information. Condition codes are not generated if they
+ * are only needed for conditional branches.
+ */
 enum {
     CC_OP_DYNAMIC, /* Use env->cc_op  */
     CC_OP_FLAGS,
diff --git a/target-i386/cpu.h b/target-i386/cpu.h
index 1cb32ae456..e64569854f 100644
--- a/target-i386/cpu.h
+++ b/target-i386/cpu.h
@@ -698,6 +698,13 @@ typedef uint32_t FeatureWordArray[FEATURE_WORDS];
 /* Use a clearer name for this.  */
 #define CPU_INTERRUPT_INIT      CPU_INTERRUPT_RESET
 
+/* Instead of computing the condition codes after each x86 instruction,
+ * QEMU just stores one operand (called CC_SRC), the result
+ * (called CC_DST) and the type of operation (called CC_OP). When the
+ * condition codes are needed, the condition codes can be calculated
+ * using this information. Condition codes are not generated if they
+ * are only needed for conditional branches.
+ */
 typedef enum {
     CC_OP_DYNAMIC, /* must use dynamic code to get cc_op */
     CC_OP_EFLAGS,  /* all cc are explicitly computed, CC_SRC = flags */
diff --git a/target-m68k/cpu.h b/target-m68k/cpu.h
index c2d40cb1cc..471f490dc1 100644
--- a/target-m68k/cpu.h
+++ b/target-m68k/cpu.h
@@ -154,6 +154,14 @@ int cpu_m68k_signal_handler(int host_signum, void *pinfo,
                            void *puc);
 void cpu_m68k_flush_flags(CPUM68KState *, int);
 
+
+/* Instead of computing the condition codes after each m68k instruction,
+ * QEMU just stores one operand (called CC_SRC), the result
+ * (called CC_DEST) and the type of operation (called CC_OP). When the
+ * condition codes are needed, the condition codes can be calculated
+ * using this information. Condition codes are not generated if they
+ * are only needed for conditional branches.
+ */
 enum {
     CC_OP_DYNAMIC, /* Use env->cc_op  */
     CC_OP_FLAGS, /* CC_DEST = CVZN, CC_SRC = unused */
diff --git a/target-s390x/cpu.h b/target-s390x/cpu.h
index 4fb34b598d..4e58cdee3e 100644
--- a/target-s390x/cpu.h
+++ b/target-s390x/cpu.h
@@ -671,6 +671,13 @@ ObjectClass *s390_cpu_class_by_name(const char *name);
 
 /* CC optimization */
 
+/* Instead of computing the condition codes after each x86 instruction,
+ * QEMU just stores the result (called CC_DST), the type of operation
+ * (called CC_OP) and whatever operands are needed (CC_SRC and possibly
+ * CC_VR). When the condition codes are needed, the condition codes can
+ * be calculated using this information. Condition codes are not generated
+ * if they are only needed for conditional branches.
+ */
 enum cc_op {
     CC_OP_CONST0 = 0,           /* CC is 0 */
     CC_OP_CONST1,               /* CC is 1 */
diff --git a/target-sparc/cpu.h b/target-sparc/cpu.h
index a3d64a4e52..646a103513 100644
--- a/target-sparc/cpu.h
+++ b/target-sparc/cpu.h
@@ -102,6 +102,11 @@
 #define CC_DST (env->cc_dst)
 #define CC_OP  (env->cc_op)
 
+/* Even though lazy evaluation of CPU condition codes tends to be less
+ * important on RISC systems where condition codes are only updated
+ * when explicitly requested, SPARC uses it to update 32-bit and 64-bit
+ * condition codes.
+ */
 enum {
     CC_OP_DYNAMIC, /* must use dynamic code to get cc_op */
     CC_OP_FLAGS,   /* all cc are back in status register */